feat: add Sqrt and SqrtRound

[mwoss]

Adds Sqrt and SqrtRound.

• SqrtRound(places) returns the square root rounded half away from zero to places digits after the decimal point, like DivRound. The result is always correctly rounded.
• Sqrt() uses DivisionPrecision, like Div.
• Both return an error for negative numbers instead of panicking.

It runs big.Int.Sqrt on the scaled coefficient and rounds once, so there are no loops in our code. That's about 4–12x faster than going through PowWithPrecision(0.5), for example 0.25 µs instead of 3.1 µs for sqrt(2).

Checked against big.Float on 30k cases, plus a property test in the repo. Builds on Go 1.10.

Replaces #393, #130 and #112. #130 and #112 give wrong digits, and #393 panics on negative numbers and is slower.