Generate non-generic write logic with source generators

[roji]

Removes a lot of ugly runtime code generation with expression trees.

I'm not completely sure if the complexity really is worth it - maybe just doing this manually everywhere makes more sense and is more maintainable (e.g. as in #3507 (review)). But this is just an opt-in mechanism which can be used where it makes sense, or the relevant code can be written manually.

Note that this doesn't cover all cases - some type handlers would have required more work to get their logic source-generated, and it didn't seem worth the effort.

Part of #2940
Part of #3300

/cc @Brar

[vonzshik]

Looks scary :D

[roji]

Yep :) But I think it's actually less scary than the previous runtime code generation - whatever happens, happens at build time and you get simple, C# sources out of it. Feels better.