The constraint (~) used to be (effectively):
  class a ~~ b => (a :: k) ~ (b :: k)

but, with this patch, it is now defined uniformly with
(~~) and Coercible like this:
  class a ~# b => (a :: k) ~ (b :: k)

Result:
  * One less superclass selection when goinng from (~) to (~#)
    Better for compile time and better for debugging with -ddump-simpl

  * The code for (~), (~~), and Coercible looks uniform, and appears
    together, e.g. in TysWiredIn and ClsInst.matchGlobalInst.
    Previously the code for (~) was different, and unique.

Not only is this simpler, but it also makes the compiler a bit faster;
  T12227: 9% less allocation
  T12545: 7% less allocation

This patch fixes Trac #15421