Switch back to `beef` for our CoW solution.

[tobz]

When we originally switched to metrics::Cow, beef didn't yet support const slicing and so we couldn't universally use it for both strings and slices. In turn, we ended up forking beef, essentially, tooling it for our specific needs.

Now that const string/slicing operations are available on stable, we should be able to theoretically switch to using beef which is a more all-encompassing solution than our hacked-and-slashed down implementation. It's an extra dependency, but it's a light dependency.

[tobz]

Playing around with this quickly: this depends on being able to specify traits on generics in const settings beyond Sized. There's no super specific issue but this is the high-level meta-issue: rust-lang/rust#57563