[MMD] Made mmd_cvt_to_types() return PMCNULL when it cannot resolve all types in a multi's signature.  In this case, you can't actually dispatch to the variant with this signature, as you don't have any parameters which correspond to the missing types -- Parrot doesn't know about them at all.  This solves the problem where aggressive multi signature type caching meant you could never dispatch to any multi variant with types you declare later, such as classes defined at runtime, after the first call to the multi.
[MMD] Made mmd_cvt_to_types() return PMCNULL when it cannot resolve all types
in a multi's signature.  In this case, you can't actually dispatch to the
variant with this signature, as you don't have any parameters which correspond
to the missing types -- Parrot doesn't know about them at all.  This solves the
problem where aggressive multi signature type caching meant you could never
dispatch to any multi variant with types you declare later, such as classes
defined at runtime, after the first call to the multi.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@32190 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

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