suspend type vars in SubTypePair's equals
suspend type vars in SubTypePair's equals

SubTypePair's equals method calls =:= on the involved types,
which mutates the TypeVars contained in them
this is undesirable since we're simply checking whether a subtype test is pending

in addition to making subtyping "more correct" for type vars,
it should avoid the stackoverflow that's been plaguing us
(https://groups.google.com/d/topic/scala-internals/2gHzNjtB4xA/discussion)

SubTypePair's equals method method is only called when subtype checking
hits a recursion threshold (subsametypeRecursions >= LogPendingSubTypesThreshold)

martijnhoekstra/scala,felixmulder/scala,scala/scala,jvican/scala,lrytz/scala,shimib/scala,felixmulder/scala,jvican/scala,felixmulder/scala,lrytz/scala,felixmulder/scala,jvican/scala,felixmulder/scala,martijnhoekstra/scala,jvican/scala,martijnhoekstra/scala,slothspot/scala,scala/scala,martijnhoekstra/scala,lrytz/scala,slothspot/scala,shimib/scala,felixmulder/scala,martijnhoekstra/scala,jvican/scala,jvican/scala,slothspot/scala,felixmulder/scala,scala/scala,slothspot/scala,shimib/scala,jvican/scala,martijnhoekstra/scala,slothspot/scala,lrytz/scala,shimib/scala,scala/scala,slothspot/scala,slothspot/scala,lrytz/scala,scala/scala,lrytz/scala,shimib/scala,shimib/scala,scala/scala