- the original exception status will be saved when entering a try: block
- sys.exc_info() will be reset after a successful except: block

This mimics the behaviour of Py3 and prevents exceptions, tracebacks
and frames (i.e. deep function state) from staying alive
any longer than necessary to handle an exception.

The new semantics imply that a try: block is no longer free, but it is still very cheap.

Ticket 65

cleanup of arg parsing code, faster string comparison in __Pyx_SplitKeywords(), some minor exception fixes

some code cleanup and beautification, fix error message to match Python when passing too many positional arguments into a function with required keyword arguments

removed some redundancy from arg parsing helper functions (and some of the helper functions)
general cleanup and performance improvements (now a lot faster for common kw passing casese)

optimisation: store keyword argument names as Python strings to avoid string creation within PyDict_GetItemString()