[WIP] SE-0492: Metatypes and existential metatypes

[kubamracek]

SE-0492 declared that constant expressions should support metatypes (of non-generic, non-resilient types). This is a sketch of implementing that support. I'm looking for feedback on whether this is the right approach, and whether we actually want to support this (emitting existential erasure of metatypes into IR constants).

[DougGregor]

This approach looks reasonable to me. I do really like having this feature, but it's a reasonable basis for any kind of "type registry" in the language