Refactoring: Split off more code from plugin/src/lib.rs into submodules

[llogiq]

The plugin source is currently split into two files, one of which is auto-generated by build.rs. The lib.rs is getting unwieldy. Time to browse for sets of functionality that can be put into their own modules, before navigating the source gets too cumbersome.

[gnieto]

Do you mean splitting each kind of mutation into a specific MultiModifier? If you don't mean this, what would you think about doing that?

[llogiq]

No, I'm rather thinking about other random stuff, like our custom TyOcc + Hash + Equals implementation. The default type whitelist stuff is likely to be removed anyway when #100 is done.

And for the record, I don't think we should split our Mutator just yet. It may make sense once we want to deactivate certain mutations by configuration, but we don't have that yet, and I'm not convinced the extra complication is worth it.

[rexto]

I'd like to help with this issue. I'm a complete beginner, I just finished reading Rust-book.