Provide some C11 generics

[embray]

C11 provides _Generic for implementing macros that provide some primitive generic multiple dispatch on type.

This would allow providing some generic calls like asdf_get (as opposed to writing asdf_get_string0 and so on) calling the appropriate implementation depending on the requested output type.

This is just syntactic sugar and not strictly necessary, but might be nice-to-have.

[embray]

I played around with this a little bit but I'm not so sure it would really be that useful after all. In particular when it comes to supporting extension types it's really tricky to make this extensible with minimal effort on the part of the extension author, limiting its usefulness. I do still have some ideas for this, but I'm just not sure it's worth spending a lot of time on right now.

[embray]

After getting some more experience I think this is still useful, even if it's hard to support extension types with it. I'm not even totally sure it's impossible--have to try it.

But what I really came here for is more interesting, which is that libfyaml does finally have support in-progress for generics, which is extensively documented here: https://github.com/pantoniou/libfyaml/blob/ec31ac1e1f7f340f29deb6399dab6d7c86732b63/doc/GENERIC-API-DESIGN.md

Unsurprisingly (there are only so many ways to do it) it works much the same as my implementation. Probably has a few strengths over it (more tightly integrated into the library, better memory handling, etc.). It also has some nice features to copy (mostly involving _Generic which was the topic of this issue).

It's also not as finnicky about ints as my implementation, but that's maybe simpler for most use cases.