Based on the original work by Justin Moon.
*MSRV unchanged from 1.29.0.*
When `std` is off, `no-std` must be on, and we use the [`alloc`](https://doc.rust-lang.org/alloc/) and core2 crates. The `alloc` crate requires the user define a global allocator.
* Import from `core` and `alloc` instead of `std`
* `alloc` only used if `no-std` is on
* Create `std` feature
* Create `no-std` feature which adds a core2 dependency to polyfill `std::io` features. This is an experimental feature and should be
used with caution.
* CI runs tests `no-std`
* MSRV for `no-std` is 1.51 or so
- Implement psbt::Map trait for psbt::Output
- Add (en)decoding logic for psbt::Output
- Implement PSBT (de)serialization trait for relevant psbt::Output types
- Add macro for merging fields for PSBT key-value maps
- Add macro for implementing decoding logic for PSBT key-value maps
- Add convenience macro for implementing both encoding and decoding
logic for PSBT key-value maps
- Add macro for inserting raw PSBT key-value pairs into PSBT key-value
maps
- Add macro for getting raw PSBT key-value pairs from PSBT key-value
maps