Split `TypeFolder` and `FallibleTypeFolder` atwain Right now there is a coherence problem with `TypeFolder` and `FallibleTypeFolder`. Namely, it's impossible to implement a `FallibleTypeFolder` that is generic over interner, b/c it has a *downstream* conflict with the blanket impl: ``` impl<I, F> FallibleTypeFolder<I> for F where F: TypeFolder<I> {} ``` Because downstream crates may implement `TypeFolder<SomeLocalInterner>` for the fallible type folder. This PR removes the relationship between `FallibleTypeFolder` and `TypeFolder`; it leads to *modest* code duplication, but otherwise does not affect perf and really doesn't matter in general.