Turn -XTypeHoles into a (on by default) warning

After some discussion on ghc-devs@ and elsewhere, it seemed favorable to
make this change as type holes don't let any invalid programs though,
they merely change what the compiler reports in case of certain errors
(namely unbound occurrences, or _ appearing on a LHS.)

Now, the warning mechanism is controlled by -f[no-]warn-type-errors,
just like any other regular warning. Again, on by default.

The documentation and tests have been updated accordingly.

Signed-off-by: Austin Seipp <austin@well-typed.com>