Interface file optimisation and removal of nameParent
This large commit combines several interrelated changes: - IfaceSyn now contains actual Names rather than the special IfaceExtName type. The binary interface file contains a symbol table of Names, where each entry is a (package, ModuleName, OccName) triple. Names in the IfaceSyn point to entries in the symbol table. This reduces the size of interface files, which should hopefully improve performance (not measured yet). The toIfaceXXX functions now do not need to pass around a function from Name -> IfaceExtName, which makes that code simpler. - Names now do not point directly to their parents, and the nameParent operation has gone away. It turned out to be hard to keep this information consistent in practice, and the parent info was only valid in some Names. Instead we made the following changes: * ImportAvails contains a new field imp_parent :: NameEnv AvailInfo which gives the family info for any Name in scope, and is used by the renamer when renaming export lists, amongst other things. This info is thrown away after renaming. * The mi_ver_fn field of ModIface now maps to (OccName,Version) instead of just Version, where the OccName is the parent name. This mapping is used when constructing the usage info for dependent modules. There may be entries in mi_ver_fn for things that are not in scope, whereas imp_parent only deals with in-scope things. * The md_exports field of ModDetails now contains [AvailInfo] rather than NameSet. This gives us family info for the exported names of a module. Also: - ifaceDeclSubBinders moved to IfaceSyn (seems like the right place for it). - heavily refactored renaming of import/export lists. - Unfortunately external core is now broken, as it relied on IfaceSyn. It requires some attention.