Caching the stable hash of Ty within itself

[oli-obk]

Instead of computing stable hashes on types as needed, we compute it during interning.

This way we can, when a hash is requested, just hash that hash, which is significantly faster than traversing the type itself.

We only do this for incremental for now, as incremental is the only frequent user of stable hashing.

As a next step we can try out

• moving the hash and TypeFlags to Interner, so projections and regions get the same benefit (tho regions are not nested, so maybe that's not a good idea? Would be nice for dedup tho)
• start comparing types via their stable hash instead of their address?