Avoid relying on OccNames when generating class methods

[crtschin]

Follows this comment by SPJ.

I'm able to simplify a bit of code following the above comment and using the class type directly instead of going through the binder environment.

• Keep a interval map for the instances in a module instead of a flat list of binders.
• Instead of using HieAST to infer the position and finding the instance declaration via AST traversal, use the cached instance map.
• Remove the hueristics on finding the right binders, by instead taking class methods and instantiating them with the instance types instead.

[soulomoon]

the idea look good to me but I have not have time to review the code yet