Bump version to 0.6.4 for fixed MSRV, errors, and perf improvements

Bump version to 0.6.4 for fixed MSRV, errors, and perf improvements

Technically this violates SemVer, but the only SemVer-breaking
change was changing some return types from `Result<T, ()>` to
`Result<T, ErrEnum>`. While someone could have been unwrap'ing the
error and storing the `()`, its sufficiently unlikely that it seems
fine to just bump the patch version to ship all the improvements
without a minor version bump.