XMLSimplifyTypes

This pass simplifies types in an XML program, eliminating all unused type arguments.

It first computes a simple fixpoint on all the datatype declarations to determine which datatype tycon args are actually used. Then it does a single pass over the program to determine which polymorphic declaration type variables are used, and rewrites types to eliminate unused type arguments.

This pass should eliminate any spurious duplication that the Monomorphise pass might perform due to phantom types.