Fix Trac #3012: allow more free-wheeling in standalone deriving

Fix Trac #3012: allow more free-wheeling in standalone deriving

In standalone deriving, we now do *not* check side conditions.
We simply generate the code and typecheck it.  If there's a type
error, it's the programmer's problem.

This means that you can do 'deriving instance Show (T a)', where
T is a GADT, for example, provided of course that the boilerplate
code does in fact typecheck.

I put some work into getting a decent error message.  In particular
if there's a type error in a method, GHC will show the entire code
for that method (since, after all, the user did not write it).
Most of the changes are to achieve that goal.

Still to come: changes in the documentation.