proposal: math: integer square root

[chiyoi]

Add a function like math.Isqrt(n int) int.

Currently I implement this by

sort.Search(n, func(x int) bool { return x * x + 2 * x + 1 > n })

but maybe this feature could be added to the standard library like math.Ilogb

[seankhliao]

our bar for what might be added to math was clarified here #58146 (comment)

c math.h doesn't have this.