Rewrite rules for Lazy.compareLength are too lazy

[clyring]

Lazy.length s must force the entirety of s, while (by design) Lazy.compareLength s 10 stops after finding at least 11 words. And yet we try to rewrite Lazy.length s > 10 to Lazy.compareLength s 10 == GT. I can't quickly think of any "reasonable-looking" example where this change will actually cause problems, but I am deeply skeptical of a rewrite rule that silently changes strictness behavior in this way, especially since it is common for forcing a lazy bytestring to mean performing deferred I/O actions.

(Also, Int64 overflow can happen in Lazy.length but not in Lazy.compareLength, though it's probably only possible to actually observe that difference in behavior by doing something rather stupid.)

[sjakobi]

Let's document that length may get rewritten to compareLength and may not force the entire string.

[Bodigrim]

My vote is for wontfix and documentation.

[clyring]

I still prefer killing the rules, but it's not a battle worth fighting. And if we aren't removing these rules then I don't consider this important for 0.12.0.