no longer compile-time-autothread operators that take Mu args.

This caused problems with the ne and !== operators, which handle
junctions manually, but also with any user-defined operator that
takes a Mu argument.

The corresponding bug is https://rt.perl.org/rt3//Public/Bug/Display.html?id=117579

I've added a few spectests to roast and ran a full spectest run, which passed successfully.