Cleans up type declarations.

This completes th type declarations changes.
ast module — last remnants of type node removed.
    adds some methods to cleanly get the declared name
    (as a String) from declaration nodes.
genc module — removed dead variable, tried (but failed) to
    add accessor method for name introduced by import … as
genus module — add accessor for name introduced by import,
    and small optimization for methods that are simple accessors.
    removes last vestiges of typenodes.
gracelib.h — removes 2 (!) declarations of ProgrammingError()
    this was introduce by me in unnecessarily in d8a383f2
identifierResolution — add decls of built-ins, removes dead code.
    tries to import StandardPrelude names in the same way as other
    modules, but eventually falls back to old code.
StandardPrelude — adds comments, changes annotation to public