Removing from the bottom of a stack (nipN(depth)) would leak the first
entry in the array by slicing [1:], leaving array[0] dangling an
inaccessible (but unable to be freed until the whole slice is gone). We
left it like this for a while, but best not to leak the memory. Happens
rarely so the performance hit shouldn't matter that much. Do the same
thing for condstack.
When we do append loops, make an educated guess as to the size and make an array
with that capacity to avoid extra copying.
Doesn't affect the speed of the tests, over 4 runs the difference was lost in
the noise.
We were counting the number of push ops instead of the number of non
push ops. Add tests that found this (checking tha the max operations
check fires).
