These new helpers make the code a lot cleaner. I confirmed that the
simple helpers like `atomic.Int64` don't add any extra overhead as they
get inlined by the compiler. `atomic.Pointer` adds an extra method call
as it no longer gets inlined, but we aren't using these on the hot path
so it is probably okay.
This PR does two things:
- Only run the tests when relevant files change.
- Cache the Go Modules directory between runs, so they don't have to redownload everything everytime (go.sum is the cache key). Pretty much straight from the examples: https://github.com/actions/cache/blob/master/examples.md#go---modules
