new exception handling semantics

new exception handling semantics
- 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.