The *name* UncheckedAnyExt was ending up visible in docs, but it had
become an increasingly unpleasant name. “Downcast” is suitable, though,
being private, it’s not still not perfect. But there’s no point in
making it public, as people generally can’t implement it because of
coherence rules (I tried).
Plus documentation and style changes.
As for Extend, eh, that should ideally be in a different commit, but
it’s here now, and I’m the only one working on this code base in
general, so I permit myself to be slightly lazy from time to time.
Trouble was Downcast should never have had an Any supertrait, as it was
grossly misleading, leading to type_id giving `dyn Any`’s TypeId rather
than the underlying type’s.