Load deprecations from interface files into a deprecation environment
which maps Names to RenamedDeprecations. This map is not used yet, but
very soon it will...

This commit fixes a bug related to implicit parameters, too:
Previously, an interface file containing the name "with" could not be
read by the interface parser. This broke Malcolm's HaXml 0.9 (released
today). Remember Sven's glaexts-commandment (Jeffrey? :-) : Always
keep Lex.lhs's ghcExtensionKeywordsFM and ParseIface.y's var_fs
production in synch!