If Unicode keys are entered to a hash, a bit is turned on.

If Unicode keys are entered to a hash, a bit is turned on.
If the bit is on, when the keys are fetched from the hash
(%h, each %h, keys %h), the Unicodified versions of the keys
are returned if needed.  This solution errs on the size of
over-Unicodifying, the old solution erred on the side of
under-Unicodifying.  As long as the hash keys can be a mix
of byte and Unicode strings, a perfect fit is hard to come by.

p4raw-id: //depot/perl@15407