Use GitHub actions to test the following axes:

• Currently stable, current beta, current nightly, MSRV stable (1.51.0)
• All of the different targets that affect us (currently, this is only relevant for the simd and simd-nightly features, which emit impls for various architecture-specific types)
• All of the Cargo features we support
• cargo check, cargo test, and cargo miri test

Note that, depending on how we implement this, some combinations of the above may not be possible to run. E.g., if we run on an x86_64 machine, it may be possible to run cargo miri test while targeting powerpc (I'm not sure about this), but it definitely won't be possible to run cargo test while targeting powerpc.

As a possible stretch goal, also include a test to make sure that README.md is kept up to date (#18).