Allow compareLength to shortcircuit in both directions

[ekmett]

This is a more invasive alternative to the patch I offered up in #75.

This permits early termination in many inexact cases (like, you know, stream!) by tracking both minimum as well as maximum sizes.

This means that examples like

>>> compareLength "hello" 6
>>> compareLength "hello" 2

can both execute in O(1), spite the fact that stream has to give an inexact bound on its size. This is because it can give both an upper bound (as usual) and lower bound (words / 2) for use in the test.

In theory more combinators could be extended to make use of the lower bounds.

I tried to exhibit care to preserve sharing like the original code, but this version could definitely use a pair of eyeballs.

If it is too much trouble then by all means just take the other version. =)

[bos]

This seems to have a significant negative impact on at least some fusion-heavy functions.

                         kmett            HEAD
Pure/toUpper/Text+tiny   449.0195   ns    357.8654   ns
Pure/toUpper/Text+ascii    4.573443 ms      4.435177 ms

Other numbers are variable in favour of your changes, so I'm not entirely sure what's going on, and am a bit inclined to just merge this.

[ekmett]

Thanks! Between these changes I should be able to viably switch trifecta over to text shortly.