IntegerSquareRoot​(System.​Numerics.​BigInteger)

The integer square root, floored. The root of the top half of the bits, shifted
back, is right to a quarter of them; one Newton step from a value on either side
lands at or above the root ((r + n/r)/2 >= sqrt(n)) with the error
squared, the next one squares it again, and the iteration then descends to the
floor and stops -- three divisions at the full width and three at a quarter of it,
where a power of two above the root took one division per bit of the exponent.