Run as annotation processor

[zml2008]

As an AP, we can share the AST that javac uses, rather than having to compile everything again for Spoon.

It appears we can continue using ASM-based class generation, though it might make sense to explore using something like [JavaPoet](https://github.com/square/javapoet) for source generation instead.

I'm unsure how incremental compilation would work -- but that could be another place to gain performance. The relevant gradle docs may help.