Rename finfer, disassemble, and methods(function,types)

Redo of #3724 using @StefanKarpinski 's names.

* finfer -> ast_typed
* methods(function, types) -> ast_lowered
* disassemble(function, types, true) -> asm_native
* disassemble(function, types, false) -> asm_llvm

`ast_lowered` is just an alias; the other three remove the method they're renaming.