Derive serde/schemars for various things

[ratmice]

This is a companion patch for linebender/kurbo#321
So, if both repos believe it to be a good idea, and wish to merge them kurbo would need to get merged first.

[ratmice]

So, I'm really not familiar with the schemars crate or json schemas in general, so schemars derive for Brush still needs to be done. I don't imagine it's hard but, but doesn't have any adaptor feature for serde_bytes, and haven't managed to figure out deriving the trait manually as I didn't manage to get it working with derive macros.

[ratmice]

The kurbo patches which were blocking this are now in, so this can perhaps make some progress.
In that spirit here are my notes on where this stands.

• I haven't tested the Blob impl, because I only needed to add it to serialize other variants of the same enum.
• BrushRef has a Serialize but no Deserialize impl, because you can't deserialize to a Non-Owned type
  Perhaps I should remove the Serialize impl from it?

Otherwise, been using it locally for a while.

[ratmice]

I'm going to just close/reopen this PR for 2 reasons:

• Mostly to just get it off of my peniko/main branch
• Because initially this added both schemars and serde, but now just contains serde as schemars support is waiting on upstream review. Thus the new branch should just mention serde, which will simplify history...