PEP 343 -- the with-statement.

This was started by Mike Bland and completed by Guido
(with help from Neal).

This still needs a __future__ statement added;
Thomas is working on Michael's patch for that aspect.

There's a small amount of code cleanup and refactoring
in ast.c, compile.c and ceval.c (I fixed the lltrace
behavior when EXT_POP is used -- however I had to make
lltrace a static global).

Parser/Python.asdl

@@ -25,6 +25,7 @@ module Python
 	      | For(expr target, expr iter, stmt* body, stmt* orelse)
 	      | While(expr test, stmt* body, stmt* orelse)
 	      | If(expr test, stmt* body, stmt* orelse)
+	      | With(expr context_expr, expr? optional_vars, stmt* body)
 
 	      -- 'type' is a bad name
 	      | Raise(expr? type, expr? inst, expr? tback)