int_sqrt: Improve 3x faster integer sqrt.

Result on 10,000,000 call.

Old:
sqrt(12345689) = 3513
real
0m0.768s
user 0m0.760s
sys 0m0.004s

New:
sqrt(12345689) = 3513
real
0m0.222s
user 0m0.224s
sys 0m0.000s

idea is taken here -
https://github.com/Lens-F/Leaping_Lemur/commit/e3dfeaccc62b2ccc51ef5378adfd05daa2b3ac35