accept unicode keys
pass the errno as positional arg too
Avoid unnecessary wrapping and unwrapping of the keys in update().  This is not JITted code.
Found another reason for slowness of dict.update(): the keys are wrapped
and unwrapped.  Fixed, and removed the magic SpecTag too.
Translation fix
Refactor the implementation of the dict iterators in RPython.  The goal
is to avoid the allocation of a tuple in iteritems().next().
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