* python/py-type.c (make_fielditem, typy_field_names, typy_items)

* python/py-type.c (make_fielditem, typy_field_names, typy_items)
(typy_length, typy_get, typy_has_key, typy_make_iter)
(typy_iterkeys, typy_iteritems, typy_itervalues, typy_iter)
(typy_iterator_iter, typy_iterator_iternext)
(typy_iterator_dealloc): New functions to implement standard
Python mapping methods on gdb.Type object.
(gdb.TypeIterator): New Python type.
* python/python-internal.h (gdbpy_iter_kind): New enum.
* doc/gdb.texinfo (gdb.Type): Document field access by dictionary
key syntax.