Raise minimum rustc version in CI to 1.56

Required by use of 2021 edition in hashbrown:

    error: failed to download `hashbrown v0.12.0`

    Caused by:
      unable to get packages from source

    Caused by:
      failed to parse manifest at `github.com-1ecc6299db9ec823/hashbrown-0.12.0/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.

Author: dtolnay

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [nightly, beta, stable, 1.52.0]
+        rust: [nightly, beta, stable, 1.56.0]
     steps:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@master