abcl-asdf: Clarify the version setting code for MVN compoments.
Fix #204, but considerably beyond the patch the Cyrus Harmon submitted. Drastically simplified the ASDF:MAYBE-PARSE-MVN function to take advantage of the fact that we are in the ASDF namespace. At least the function now makes sense Parts of ASDF *reallY* want ASDF:VERSION to be a triple of intergers, and never anything more, which is part of the reason for the shennigans here.
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