Adding the `TypeId` FFI trait

[bendk]

The metadata code now uses this trait to get the TYPE_ID_METADATA rather than Lift/Lower. This feels cleaner, since it was often not clear which trait to use before. This removes the hacky handling of type id metadata for callback interfaces that I added in #1950.

Let's not merge this until after 0.26.0 is released. It feels like too big of a change at this point in the process.