The typechecking of arrow forms (in GHC 7.6) is known to be bogus, as
described in Trac #5609, because it marches down tuple types that may
not yet be fully worked out, depending on when constraint solving
happens.  Moreover, coercions are generated and simply discarded.  The
fact that it works at all is a miracle.

This refactoring is based on a conversation with Ross, where we
rearranged the typing of the argument stack, so that the arrows
have the form
   a (env, (arg1, (arg2, ...(argn, ())))) res
rather than
   a (arg1, (arg2, ...(argn, env))) res
as it was before.

This is vastly simpler to typecheck; just look at the beautiful,
simple type checking of arrow forms now!

We need a new HsCmdCast to capture the coercions generated from
the argument stack.

This leaves us in a better position to tackle the open arrow tickets
 * Trac #5777 still fails.  (I was hoping this patch would cure it.)
 * Trac #5609 is too complicated for me to grok.  Ross?
 * Trac #344
 * Trac #5333