Extraction: avoid some useless Obj.magic by fixing my ML type unifier
Extraction: avoid some useless Obj.magic by fixing my ML type unifier

 Due to wrong pattern order in Mlutil.mgu, simple situations like
 ?n == ?n were considered unsolvable as soon as one side was aliased
 (i.e. inside an instantiated type meta).

 Moreover, use general equality as last resort, instead of forgetting
 cases like Taxiom == Taxiom.

git-svn-id: 44dc50bdcc0f5d1a212c497508dc9452e0b35e2b@13963 85f007b7-540e-0410-9357-904b9bb8a0f7

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