Restrict const generic types

[CohenArthur]

Per the documentation on const generics:

The only allowed types of const parameters are u8, u16, u32, u64, u128, usize i8, i16, i32, i64, i128, isize, char and bool.

Afterwards, we can look into more complex types gated behind the adt_const_params feature

[CohenArthur]

I'm unsure as to whether this should live in our typechecker or our const checker. One one hand, it makes sense to check that during typechecking, on the other hand it could be quite a large amount of code + we'd need to handle the feature properly. I think it would be fine to put that as part of the const checker, and lookup types using the type context (and disable that error if the feature is enabled)