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use "as" for block scope support
just like mozilla javascript "let" extension
https://developer.mozilla.org/en/new_in_javascript_1.7#Block_scope_with_let_%28Merge_into_let_Statement%29
usage:
as_clause(as_declare): "as" NAME
as_stmt: NAME "as" test
as_expr: "(" expr "as" NAME ")" expr
var declare by as_clause is alive in <suite> only
eg:
<NAME "f" binding saved>with open(...) as f<NAME "f" bind to HEAPObj>:
 ...
<NAME "f" binding restored>
the same rule to "except E as e: "
spec:
from socket import AF_INET as IPv4
<IPv4 bind to socket.AF_INET slot>
IPv4 = None
assert socket.AF_INET == None
var in as_stmt is alive in current indentation
eg:
for i in range(10):
 <NAME "t" binding saved>
 t as f()+1 <NAME "t" bind to f()+1>
 ...
 <NAME "t" binding restored>
var in as_expr is alive in sub expr only
eg:
t = None
x = (2+3 as t) t**2
assert t == None ane x == 25
Suggestion:
new opcode
ENTER_BLOCK #depth #varslots