Merge ast-branch to head

This change implements a new bytecode compiler, based on a
transformation of the parse tree to an abstract syntax defined in
Parser/Python.asdl.

The compiler implementation is not complete, but it is in stable
enough shape to run the entire test suite excepting two disabled
tests.

Lib/test/test_generators.py

@@ -774,7 +774,7 @@ These are fine:
 ...         try:
 ...             1//0
 ...         except ZeroDivisionError:
-...             yield 666  # bad because *outer* try has finally
+...             yield 666 
 ...         except:
 ...             pass
 ...     finally: