Undeprecate the MAYBE_FLAT bit, which (contrary to my mistaken memory) is not unused, but is in fact very often used for Perl 6.
Undeprecate the MAYBE_FLAT bit, which (contrary to my mistaken memory) is
not unused, but is in fact very often used for Perl 6.

New features: (1) When a hash value is flatted for MAYBE_FLAT, it must be
flattened as a list of pair PMCs; and (2) the proper pair PMC class to use
for this purpose must be indexed by HLL.


git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@11894 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

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