Test monomorphized AST

[katat]

After monomorphization phase, it will generate a monomorphized AST (MAST).

To ensure the monomorphization process works as expected, it'd better to have snapshot tests for the generated MASTs for different cases.

There is some tool like https://github.com/mitsuhiko/insta?tab=readme-ov-file to do the job. But I think we should be able to structure the snapshot tests small enough, so the generated AST string can be compared in a manageable way.