Add valuable support

[KodrAus]

It's not currently released let alone stable yet, but the valuable crate looks like it should be a good candidate for inclusion in value-bag. I've done some exploration on bridging valuable with serde and sval and so far that seems entirely possible. We do have a compounding maintenance cost in introducing new erasure frameworks, but valuable is one that seems worth the cost.

[KodrAus]

There's a 0.1.0 of valuable now that we can start to look at. I've been tinkering with valuable support for sval, which works out just fine. As for serde, I think there's still some more space to explore between them. The easiest to integrate valuable with would probably be std::fmt.