Loopy: revamp loop detection, but not using findloop.

Loopy: revamp loop detection, but not using findloop.

Loopy differs from the other recently-reworked puzzles in that it
doesn't disallow loops completely in the solution - indeed, one is
actually required! But not all loops are what you wanted, so you have
to be a bit more subtle in what you highlight as an error. And the new
findloop system doesn't make that easy, because it only answers the
question 'is this edge part of a loop?' and doesn't talk about loops
as a whole, or enumerate them.

But since I was working in this area anyway, I thought I might as well
have a think about it; and I've come up with a strategy that seems
quite sensible to me, which I describe in a big comment added in
loopy.c. In particular, the new strategy should make a more sensible
decision about whether to highlight the loop or the non-loop edges, in
cases where the user has managed to enter a loop plus some extra
stuff.