Refined type hierarchy of dictionary iterators

Hi all,

I refined dicitonary iterator types a bit:
https://github.com/jythontools/jython/commit/5b1ed9b0e42dcaa21549ea4a10ee9136c1fb426e

Goal is to allow an easier support for iterator types in JyNI
(cf. https://github.com/Stewori/JyNI/issues/13).

This is fairly internal stuff and does not affect public API, so it shouldn't break
anything that is "properly" using Jython API. However, we all know that internal API
sometimes is not as internal as one might expect in Java. So, if somebody sees an
issue with this, speak up now ;)

Main design goal of this change:

- Let iterators for values, keys and items be of distinct classes
  (because CPython has it this way and then it's easier to match in JyNI)
- Let the same kind of such iterators from PyDictionary and PyStringMap
  have a common superclass (defined in AbstractDict)

Best,

-Stefan

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot