Trac #14218 showed that we were not solving implicit-parameter
constraints correctly.  In particular,

- A tuple constraint could "hide" an implicit-parameter wanted
  constraint, and that in turn could that we solved it from the
  wrong implicit-parameter binding.

- As a special case the HasCallStack constraint (which is just
  short for (IP "callStack" CallStack), was getting mis-solved.

The big change is to arrange that, in TcSMonad.findDict when looking
for a dictionary, either when looking for a matching inert or solved
dictionary, we fail for

  - Tuples that are hiding implicit parameters
    See Note [Tuples hiding implicit parameters]

  - HasCallStack constraints where we have not yet pushed
    on the call-site info
    See Note [Solving CallStack constraints]

I also did a little refactoring

* Move naturallyCoherentClass from Class to TcInteract, its sole
  use site.  Class.hs seems like the wrong place.  (And I also
  do not understand the reason that we need the eq/Coercible/
  Typable stuff in this predicate, but I'll tackle that separately.)

* Move the code that pushes call-site info onto a call stack
  from the "interact" part to the "canonicalise" part of the solver.