This can be seen as using the input itself as an implicit
queue; we formerly copied its chunks into an explicit queue.

By writing the key logic as a polymorphic `splitAtEndFold`,
it was easy to re-use it for `takeEnd`; the latter function
should now operate in constant stack space and can release
initial chunks of a very long input string sooner.

...so that a plain 'cabal test' finds the bug almost every try
instead of finding it only every few dozen tries

(Some re-compilation check somewhere set a trap for me.)

This also replaces fromIntegral with intToIndexTy in a few places.