Rust version 1.56.0 introduced edition 2021. Shortly afterwards, on
October 21 2021 Rust version 1.56.1 was released.
Debian stable is currently shipping `rustc 1.63.0`.
Our stated MSRV policy is: In Debian stable and at least 2 years old.
Therefore our MSRV policy is met by Rust version 1.56.1 and we can strat
to bump our MSRV org wide.
Start by bumping the `rust-bitcoin` and `hashes` MSRV to Rust 1.56.1,
includes:
- Update docs.
- Update CI and remove pinning.
- Update the build files and remove now stale cfg attributes rust_v_1_x
for values less than the new MSRV.
- Use new `IntoIterator` for arrays so we no longer need to allocate a
vector to iterate.
Links:
- https://blog.rust-lang.org/2021/11/01/Rust-1.56.1.html
- https://blog.rust-lang.org/2021/10/21/Rust-1.56.0.html
- https://packages.debian.org/stable/rust/rustc
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