Instead of the previous unreliable hack, actually look for occurrences
of the solved metavariable. Block if there are any, and fail if there
are any under a constructor. I had expected this to hurt performance
quite a bit, but it seems it doesn't.
This was prompted by #304, which is now partly fixed, but there's still
a refinement to unification needed to fix it fully (and at least it
doesn't segfault due to the cycle now!)
This slows things down a bit because to find the holes and give them the
right multiplicities, we need to normalise all the arguments which might
have been metavariables. Maybe we should skip this if we're not using
anything linear, for efficiency?
As patterns are handled by deciding which side of the as is considered
'used'. In case blocks, that should be the variable name, but in general
it should be the pattern, so IAs now has a flag to say which one.
