Very rough first draft at codegen module

[ratmice]

Here is a (not very pretty) first draft which is mostly just pulling code as-is out of CTLexerBuilder::build() into smaller, more manageable functions. I don't think anything here is really properly named, codegen at times seems like it's doing more parsing/validation than actual code generation (where it never uses it's self type!)

I thought that the splitting of the CTTokenMapBuilder turned out okay, however it is quite a bit simpler with less moving parts.

My thoughts are to try and rename the LexCodegen structure to something like LexCodegenBuilder,
and then try see if we can manage to come up with a good Self type for the codegen parts.

Edit:
I haven't tried to make anything public here yet, aiming for close to verbatim code movement.
I do have an idea of what a second draft might look like, we'll see if it works soon.