Merge bitcoindevkit/bdk#1077: fix: msrv issues

b0b91b74186575f11a4fba77ffd64005c482740e fix: msrv issues and update coverage toolchain (Vladimir Fomene)

Pull request description:

  Fix breaking MSRV for 1.57.0

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  danielabrozzoni:
    ACK b0b91b74186575f11a4fba77ffd64005c482740e

Tree-SHA512: d872f427da065c66346f0fd3ae19d4d62c535cf4444d236c0d1f6e9a5e169b7fd7773f643693daa23eeccbe6a3ddad6767f91520c7547360c07febc227a88f06