[Compiler-sig] ast branch

[Neal Norwitz]

The AST branch seems to be making pretty good progress.  I just wanted
to collect the known issues so we don't overlap.  Brett Cannon had
mentioned that he was interested in the new compiler, so perhaps this
will provide some insight into the status.

Known problems:

        * class is not handled
        * need more error checking on memory alloc failures, etc.
        * use PyOS_snprintf instead of sprintf
        * the following code at the module scope core dumps:

             if 1:
                 try:
                     from posix import _exit
                 except ImportError:
                     pass


          there is an extra POP_TOP at offset 10 after SETUP_EXCEPT
          the initial JUMP_IF_FALSE jumps into the try, not after

The new files are in Python: asdl.c, ast.c newcompile.c.
Python/symtable.c has been modified to use the new AST too.

I'm sure there's a ton of things I forgot or don't know.

Neal