std.math: add fns degrees, radians, isqrt and fract

degrees and radians
Converting to and from degrees and radians is a common math task so these two could be really handy.

isqrt
I know that math.sqrt can return an int sqrt but it's more clear if there's something that indicates it. And an int sqrt isn't "exactly" a sqrt. It's more like a floored sqrt.

fract
It returns the fractional part(e.g. .5 from 1.5).
This is really just for convenience. Not as significant as the other three.

[nektro]

implementation is here https://github.com/ziglang/zig/blob/master/lib/std/math/sqrt.zig#L7-L34

I already saw that. That's why I said "I know that math.sqrt can return an int sqrt". What follows after that is the reason.

[nektro]

the implementation supports floats too. that's why It doesn't have any signifying information