The major change in this version is the increase of the Minimum Supported Rust Version (MSRV) from 1.29 to 1.41.1, this is a big change because it introduces Rust Edition 2018 to the codebase along with all the benefits that brings.
## Breaking changes
* [MSRV bumped to 1.41.1 and edition changed to 2018](https://github.com/rust-bitcoin/rust-secp256k1/pull/331)
* serde implements serialize [fixed-width arrays](https://github.com/rust-bitcoin/rust-secp256k1/pull/406) as tuples in binary formats to avoid encoding the length
* Key tweaking methods renamed and refactored to use a more [functional-style](https://github.com/rust-bitcoin/rust-secp256k1/pull/406), they now accept a [new Scalar](https://github.com/rust-bitcoin/rust-secp256k1/pull/445) type instead of raw slices
* Update [`rand` dependency to 0.8](https://github.com/rust-bitcoin/rust-secp256k1/pull/331)
*`KeyPair::from_secret_key` [borrows SecretKey](https://github.com/rust-bitcoin/rust-secp256k1/pull/430) instead of taking ownership
## New features/APIs
* Several convenience/conversion methods between [keys](https://github.com/rust-bitcoin/rust-secp256k1/pull/430)
* [`sign_ecdsa_with_noncedata` and `sign_ecdsa_recoverable_with_noncedata`](https://github.com/rust-bitcoin/rust-secp256k1/pull/425) can be used to add additional entropy to ECDSA signatures
* Errors now display their sources if std is unavailable, with std they rely on the consumer to use the source() method
* [Implemented `TryFrom` for `Parity`](https://github.com/rust-bitcoin/rust-secp256k1/pull/409)
* The [alloc feature](https://github.com/rust-bitcoin/rust-secp256k1/pull/331) can be used on targets with allocators without a standard library
*`SharedSecret` can be created from a slice, parsed from a hex string, or [(de)serialized using serde](https://github.com/rust-bitcoin/rust-secp256k1/pull/418)
* We now [derive `std::hash::Hash` for `Signature`](https://github.com/rust-bitcoin/rust-secp256k1/pull/441)
## Other improvements
*`global-context` feature now activates `global-context-less-secure`.
*`githooks/` directory added for contributors
* [Clippy is now used in CI](https://github.com/rust-bitcoin/rust-secp256k1/pull/448) and the code is clippy-compliant
* [Simplify `SharedSecret` API](https://github.com/rust-bitcoin/rust-secp256k1/pull/402) to use a 32-byte buffer; users of custom hashes should now use bare arrays rather than this type.
* Change [serde serialization of `Parity`](https://github.com/rust-bitcoin/rust-secp256k1/pull/401) from `i32` to `u8`; clean up [error handling](https://github.com/rust-bitcoin/rust-secp256k1/pull/403)
* Deprecate the [`generate_schnorrsig_keypair` method](https://github.com/rust-bitcoin/rust-secp256k1/pull/372) (unclear value)
* Add [serde traits to `KeyPair`](https://github.com/rust-bitcoin/rust-secp256k1/pull/379)
* Redo the [API of the new `Parity` type](https://github.com/rust-bitcoin/rust-secp256k1/pull/382) to more clearly match our desired semantics; **the `From<i32>` impl on this type is now deprecated**. Also [#400](https://github.com/rust-bitcoin/rust-secp256k1/pull/400).
* Randomize [the global context on creation](https://github.com/rust-bitcoin/rust-secp256k1/pull/385) when possible; weaken [`global-context-less-secure` feature accordingly](https://github.com/rust-bitcoin/rust-secp256k1/pull/407).
* Improve [the global context API](https://github.com/rust-bitcoin/rust-secp256k1/pull/392)
* Fix [the `Debug` impl](https://github.com/rust-bitcoin/rust-secp256k1/pull/393) for `RecoverableSignature`
* Implement [`LowerHex` and `Display`](https://github.com/rust-bitcoin/rust-secp256k1/pull/398)
* Increase [`bitcoin_hashes` version to 0.10](https://github.com/rust-bitcoin/rust-secp256k1/pull/326); rename `secp256k1::bitcoin_hashes` module to `secp256k1::hashes` to align with `bitcoin` crate naming
* Add new [error variant for `PublicKey::combine_keys`](https://github.com/rust-bitcoin/rust-secp256k1/pull/304)
* Change `Display` and `Debug` for secret keys to [only output a truncated hash](https://github.com/rust-bitcoin/rust-secp256k1/pull/312)
* Fix [context bound requirements for a few methods](https://github.com/rust-bitcoin/rust-secp256k1/pull/342)
* Add a [static immutable-zero aligned type](https://github.com/rust-bitcoin/rust-secp256k1/pull/345)
* Change `tweak_add_assign` and `tweak_add_check` to [use an opaque `Parity` type rather than a boolean](https://github.com/rust-bitcoin/rust-secp256k1/pull/344/)
* Fix [`SecretKey` validation in `from_str`](https://github.com/rust-bitcoin/rust-secp256k1/pull/296)
* Add [`global-context-less-secure` feature](https://github.com/rust-bitcoin/rust-secp256k1/pull/279) which creates a non-randomized global context (and does not require `rand` or `std`)
* [remove `ffi::PublicKey::blank`](https://github.com/rust-bitcoin/rust-secp256k1/pull/232) and replace with unsafe [`ffi::PublicKey::new` and `ffi::PublicKey::from_array_unchecked`](https://github.com/rust-bitcoin/rust-secp256k1/pull/253/); similar for all other FFI types
* [make the global-context feature depend on the rand-std feature](https://github.com/rust-bitcoin/rust-secp256k1/pull/246)
* [add a lexicographic ordering to `PublicKey`](https://github.com/rust-bitcoin/rust-secp256k1/pull/248) which does **not** match the ordering used by Bitcoin Core (matching this would be impossible as it requires tracking a compressedness flag, which libsecp256k1 does not have)
* [require use of new `AlignedType` in preallocated-context API to enforce alignment requirements](https://github.com/rust-bitcoin/rust-secp256k1/pull/233); previously it was possible to get UB by using misaligned memory stores
* [enforce correct alignment when using preallocated context API](https://github.com/rust-bitcoin/rust-secp256k1/pull/233)
* [stop using cargo features for dangerous build-breaking options, require setting `RUSTFLAGS` instead](https://github.com/rust-bitcoin/rust-secp256k1/pull/263)
* [implement low-R signing and function to grind even smaller signatures](https://github.com/rust-bitcoin/rust-secp256k1/pull/259)
* [remove endomorphism feature, following upstream in enabling it by default](https://github.com/rust-bitcoin/rust-secp256k1/pull/257)
* A [complete API overhaul](https://github.com/rust-bitcoin/rust-secp256k1/pull/27) to move many runtime errors into compiletime errors
* Update [libsecp256k1 to `1e6f1f5ad5e7f1e3ef79313ec02023902bf8`](https://github.com/rust-bitcoin/rust-secp256k1/pull/32). Should be no visible changes.
* [Remove `PublicKey::new()` and `PublicKey::is_valid()`](https://github.com/rust-bitcoin/rust-secp256k1/pull/37) since `new` was unsafe and it should now be impossible to create invalid `PublicKey` objects through the API
* [Reintroduce serde support](https://github.com/rust-bitcoin/rust-secp256k1/pull/38) behind a feature gate using serde 1.0