make the NULL-instead-of-None change in tp_clear() optional and off by default - breaks too much code for now

Assignmment based type inference

no_gc_clear implementation (disable tp_clear) + doc and tests

Per Stefan's comment these are unneeded and actually the tests still pass.

This can be useful if some external objects have to be cleaned up when
the Python object dies. Otherwise the cyclic garbage collector may
drop the references before the cleanup has any chance to run.

Includes documentation and a simple unit test.

Discussion: http://article.gmane.org/gmane.comp.python.cython.devel/14986

Better to crash early than to never know that the reference changed to
None. See discussion here:

http://article.gmane.org/gmane.comp.python.cython.devel/14833

properly handle expressions at the beginning of func/class/etc. blocks that start with a string literal but are not docstrings