Merge TcTypeableValidity into TcTypeable, document treatment of casts

This patch:

• Implements a refactoring (suggested in !1199 (closed) (comment 207345)) that moves all functions from TcTypeableValidity back to TcTypeable, as the former module doesn't really need to live on its own.
• Adds Note [Typeable instances for casted types] to TcTypeable explaining why the Typeable solver currently does not support types containing casts.

Resolves #16835 (closed).