Add dynamically-linked plugins (see Trac #3843)
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.