Properly handle cuts in LHS of flatMap calls and Not/Lookahead/Filter

[lihaoyi]

We were not properly propagating the cuts in the LHS of flatMaps; that meant that if there was a cut inside the left hand parser, it doesn't take effect if something fails in the RHS of the flatMap call, or if the flatMap succeeds but something fails later.

Also makes ! and & negative/positive lookaheads behave as NoCuts: the whole point of those operators is to backtrack, so there isn't any conceivable use-case where someone would want to use them and have a cut disable-backtracking/drop-buffer when inside one of these operators.

Also properly propagates cuts in foo.filter(pred) when foo passes but pred fails.

Found these when investigating the build failure in #122. Possibly not the only issue, but it's something. This changes behavior, but if this makes any tests fail, I'll fix them.

Review by @vovapolu

[lihaoyi]

In theory, this should be all that is necessary to make #122 pass