Implement base type-level interpreter and subtyping

[CohenArthur]

• [wip]: Start working on type var interpreter. Bring over UnionType from FIR
• [wip]: Go further with the typelevel interpreter
• [wip] it compiles
• [wip]: Improve errors and API
• wip: Add proper handling of Kind::UnionType
• [wip] almost there
• [wip] it fucken works ~ ! ! ~
• typecheck: Start adding TypeMap data structure
• typecheck: Start making TypeCtx generic

[CohenArthur]

• Make Actual transform the old typectx to the new one properly

Once this is done we can probably merge this

• Once the first phase of flattening is done, add more flattening within each type and garbage collect? which will make comparisons easier
• Or should that be done as we build the list?

[CohenArthur]

So what we actually want is to have something like the following API:

fn insert_type(key: OriginIdx, ty: Type) -> TypeReference {}
fn insert(key: OriginIdx, tyref: TypeReference) {}

[CohenArthur]

{
  to_id -> ref int,
  x -> ref int,
  y -> ref int,
  z -> ref int,
}

+

{ ref int -> int }