GitLab

This is a little nicer than having to explicitly split supplies and
throw half of them away.

Lots of comments with decomposeRuleLhs

We generate intermediate unfoldings which are just thrown away, so
evaluating them is a waste of time.

Update the documentation on using DLL's from Windows, fixing several errors, in particular those relating to bug 3605

(1) More lenient kind checking, (2) Fixed orientation problems and avoiding double unifications, (3) Comments

We weren't doing fundeps for derived superclasses

Fixes a loop in the compiler, when running the dph tests

Contrary to the comment, for the module in #4367 at least, it is a big
improvement. Without it we get a huge spike of drag.

A bit yukky; see Note [Runtime skolems] in TcErrors.
But it works, and the debugger just is yukky in places.

In particular there is much less fiddly skolemisation now
Things are not *quite* right (break001 and 006 still fail), 
but they are *much* better than before.

Major pass through type checker:(1) prioritizing equalities, (2) improved Derived mechanism, (3) bugfixes

(1) Caching FD improvements for efficiency, (2) preventing cascading deriveds from entering the inert, (3) Fixing bugs in the creation of FlexiTcS variables