Use memory arena for types when type checking

[lpil]

Currently, we represent types using a nested tree structure in which nodes are reference counted. This is at times awkward to work with, not particularly well-suited to a multithreaded compilation, and potentially slower than using an arena due to the overhead of reference, counting and memory fragmentation. Use an id based memory arena instead.

It is only during type interference and checking that we need to mutate types and to have the various different kinds of type variables, so for all other parts of the compiler, we can have a different, simpler data structure with which to represent types. Related to https://github.com/gleam-lang/gleam/issues/865

[abs0luty]

I'm working on it rn. Unfortunately, there are a lot of uses of Arc<Type> in the compiler's source code, so keeping up with updates on the main branch will be hard, but I will try.

[lpil]

Thank you! I expect the main branch will be pretty quiet for a while as I'm focusing on documentation, so now is a good time.

Note one thing we'd like to do is be able to use multiple threads, so an arena where the storage can be sent to another thread such as id arena would be best.

[abs0luty]

@lpil I use id_arena crate as for memory arena for patterns in exhaustive type checking, so should be fine.