deprecates macro def return type inference

With the new focus on quasiquotes in macro implementations, we now have
to change the way how inference of macro def return types works.

Previously, if the return type of a macro def wasn’t specified, we looked into
the signature of its macro impl, took its return type (which could only
be c.Expr[T]) and then assigned T to be the return type of the macro def.

We also had a convenient special case which inferred Any in case when
the body of the macro impl wasn’t an expr. That avoided reporting spurious
errors if the macro impl had its body typed incorrectly (because in that
case we would report a def/impl signature mismatch anyway) and also provided
a convenience by letting macro impls end with `???`.

However now we also allow macro impls to return c.Tree, which means that
we are no longer able to do any meaningful type inference, because c.Tree
could correspond to tree of any type.

Unfortunately, when coupled with the type inference special case described
above, this means that the users who migrate from exprs to quasiquotes
are going to face an unpleasant surprise. If they haven’t provided
explicit return types for their macro defs, those types are going to be
silently inferred as `Any`!

This commit plugs this loophole by prohibiting type inference from
non-expr return types of macro impls (not counting Nothing). Moreover,
it also deprecates c.Expr[T] => T inference in order to avoid confusion
when switching between exprs and quasiquotes.