math/big: add Float.Sqrt

[ALTree]

From #14102

Add a Float method to compute square roots:

// Sqrt sets z to the square root of x, and returns z.
func (z *Float) Sqrt(x *Float) *Float

[rsc]

Thanks for creating the issue. If you'd like to work on this, I highly recommend Brent and Zimmerman, Modern Computer Arithmetic, available online at https://members.loria.fr/PZimmermann/mca/mca-cup-0.5.9.pdf. This is what I used for integer square root and some other things, and it looks like floating-point square root is covered as well.

As far as the API is concerned, I'm OK with the addition but will leave the final decision to Robert.

/cc @griesemer

[griesemer]

I'm ok with sqrt for big.Floats. @ALTree if you plan to work on this, please implement this in a new file. Also, it may take a while before I get to reviewing it.

[ALTree]

Thanks for the suggestions.

@griesemer no problem, there's plenty of time before the go1.10 freeze.

[gopherbot]

Change https://golang.org/cl/67830 mentions this issue: math/big: add (*Float).Sqrt