Raise minimum tested toolchain to rust 1.56
Raise minimum tested toolchain to rust 1.56

Required by the transitive dev-dependency on once_cell.

    error: failed to parse manifest at github.com-1ecc6299db9ec823/once_cell-1.15.0/Cargo.toml

    Caused by:
      feature `edition2021` is required

      The package requires the Cargo feature called `edition2021`, but that feature is not stabilized in this version of Cargo (1.55.0 (32da73ab1 2021-08-23)).
      Consider trying a newer version of Cargo (this may require the nightly release).
      See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#edition-2021 for more information about the status of this feature.

dtolnay/thiserror