Various fixes for AOT compilation

[Cidolfas]

I was evaluating Ceras for a project that's currently still on Unity 2019.4 in .NET standard 2.0 mode, and I figured I'd go ahead and share all the work I had to do to get AOT compilation to work properly for it.

Most of these things are just small bugs, but a few of them veer into new feature territory (like generating formatters for subclasses of an abstract base class).

I don't expect to do much more work here, and since this repo hasn't been updated in a few years I'm mostly just leaving it for anybody in the future who runs into these issues.