Implement Value for Cow<T> where T: Value

[loewenheim]

This implements Value for Cow<'a, T> where T: Value.

Motivation

This is one part of #726. I also recently ran into this personally when trying to record a field of type Option<Cow<str>>.

[joshka]

Can this be unit tested reasonably?

[loewenheim]

Can this be unit tested reasonably?

I could adapt a test like this one to use Cow, would that be sufficient?

[joshka]

I could adapt a test like this one to use Cow, would that be sufficient?

Yeah, probably. It's likely no big deal, as this code is super simple. But a unit test here helps clarify the behavior and catches any future regressions.