Add structured support to kv::Value using sval

[KodrAus]

What's this all about?

This PR adds a new unstable feature for structured logging that lets consumers actually interact with the structure of values.

It doesn't do this by exposing its own serialization contract. Instead, it implements traits from a serialization framework called sval. I'm open to any philosophical debate on different approaches here!

There are two commits that I think demonstrate how it fits together:

• Refactor value internals so we can add in-band serialization frameworks easily.
• Add sval support. This is the one to look at if you're interested in what impact introducing in-band frameworks has.

What is sval?

sval is a serialization-only framework I've been working on specifically for use in log. sval supports no-std, object-safe serialization for primitives and more complex structures like maps and sequences. It absolves log from having to produce and maintain its own serialization contract, so it can stay focused, but lets it provide a complete serialization API for consumers.

What about serde?

serde is the other serialization framework I've been planning to support in-band in log because it's ubiquitous (and does its job very well). I haven't started with it because:

• It'll require std to work, because we need erased-serde.
• sval has out-of-the-box support for serde (including in no-std environments) so adding it first already gives consumers a way to capture implementations of serde::Serialize.

The plan is to also eventually offer serde support.

Impact on build times

When sval is enabled through the kv_unstable_sval feature, it increases our compile times by a little bit:

log Features	Dependencies	fresh cargo build time (best of 5 runs)
kv_unstable	none	1.28s
kv_unstable_sval	sval	1.47s