This patch uses CallStack support in GHC 7.10 and GHC 8.0 to make
it possible to get stack traces when we log output.

This is the bare minimum to make this patch useful: there is
plenty of tuning that can be done.  For example:

* Insertions of withFrozenCallStack can help make the "callsite" output
  more useful, though be careful, we lose all stack information at that point!

* Insertions of 'WithVerbosity', which will let us get deeper stacks
  (at the moment, they are basically always 1-deep.)

Fixes #3768.

CC @23Skidoo



Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>