Tidying up Core

This module's purpose is to prepare the Core program for two distinct purposes: * To be serialised into the module's interface file * To feed to the code generator

The most important tasks are: * Determine which Names should ultimately be Internal and External (which may differ to whether they were originally Internal or External). See `Note [About the NameSorts]` in GHC.Types.Name. For example, in: module M where f x = x + y where y = factorial 4 could be optimized during the Core pass to: module M where y = factorial 4 f x = x + y in which case y would be changed from Internal to External.

• Rename local identifiers to avoid name clashes, so that unfoldings etc can be serialialised using the OccName, without Uniques.

For example (x_5 means x with a Unique of `5`): f x_12 x_23 = x_12 would be changed to: f x_12 x1_23 = x_12