Implement sqrt function

[Pzixel]

There is currently no SQRT function which is critically important to have. Multiple algorithms on arbitrary-precision types require it. I think it worth to implement it in the crate, and as a result - in depended types, such as Ratio.

[cuviper]

It's coming here -- see #51.

Ratio will be trickier to deal with precision, because you probably don't want to just round √(a/b) by operating on the numerator and denominator separately. That is, ⌊√a⌋/⌊√b⌋ would be quite lossy.
(But we should take that discussion to the num-rational repo.)

[Pzixel]

Sorry, I've checked that there is not open discussion about it, but I didn't check open PRs. Shame on me 🙂