Test with Rust 1.56 in CI

This our current minimum supported Rust version (MSRV). I gave up on
maintaining compatibility with Rust 2018 two years ago in #180, but I
hope the update speed of our dependencies has slowed down enough that
we can keep compatibility with Rust 1.56 for a while.

If this turns out to be problematic again, then I will bump the MSRV
as needed. This will of course be clearly communicated in the release
notes for each release.

.github/workflows/build.yml

@@ -22,6 +22,7 @@ jobs:
  strategy:
  matrix:
  rust:
+ - 1.56 # MSRV, Rust 2021
  - stable
  - nightly
  features: