This patch was originally developed by Max Bolingbroke, and worked on
further by Austin Seipp.  It allows you to write a Core-to-Core pass
and have it dynamically linked into an otherwise-unmodified GHC, and
run at a place you specify in the Core optimisation pipeline.

Main components:
  - CoreMonad: new types Plugin, PluginPass
               plus a new constructor CoreDoPluginPass in CoreToDo

  - SimplCore: stuff to dynamically load any plugins, splice
    them into the core-to-core pipeline, and invoke them

  - Move "getCoreToDo :: DynFlags -> [CoreToDo]"
      which constructs the main core-to-core pipeline
      from CoreMonad to SimplCore
    SimplCore is the driver for the optimisation pipeline, and it
    makes more sense to have the pipeline construction in the driver
    not in the infrastructure module.

  - New module DynamicLoading: invoked by SimplCore to load any plugins
    Some consequential changes in Linker.

  - New module GhcPlugins: this should be imported by plugin modules; it
    it not used by GHC itself.