Fix 2030: make -XScopedTypeVariables imply -XRelaxedPolyRec

The type checker doesn't support lexically scoped type variables
unless we are using the RelaxedPolyRec option.  Reasons: see
Note [Scoped tyvars] in TcBinds.

So I've changed DynFlags to add this implication, improved the
documentation, and simplified the code in TcBinds somewhat.
(It's longer but only because of comments!)