While trying to fix #1666 (-Werror aborts too early) I decided to some
tidyup in GHC/DriverPipeline/HscMain.

 - The GhcMonad overloading is gone from DriverPipeline and HscMain
   now.  GhcMonad is now defined in a module of its own, and only
   used in the top-level GHC layer.  DriverPipeline and HscMain
   use the plain IO monad and take HscEnv as an argument.

 - WarnLogMonad is gone.  printExceptionAndWarnings is now called
   printException (the old name is deprecated).  Session no longer
   contains warnings.

 - HscMain has its own little monad that collects warnings, and also
   plumbs HscEnv around.  The idea here is that warnings are collected
   while we're in HscMain, but on exit from HscMain (any function) we
   check for warnings and either print them (via log_action, so IDEs
   can still override the printing), or turn them into an error if
   -Werror is on.

 - GhcApiCallbacks is gone, along with GHC.loadWithLogger.  Thomas
   Schilling told me he wasn't using these, and I don't see a good
   reason to have them.

 - there's a new pure API to the parser (suggestion from Neil Mitchell):
      parser :: String
             -> DynFlags
             -> FilePath
             -> Either ErrorMessages (WarningMessages, 
                                      Located (HsModule RdrName))