rotate
Rotates the binary representation of this UInt128 number left by the specified bitCount number of bits. The most significant bits pushed out from the left side reenter the number as the least significant bits on the right side.
Rotating the number left by a negative bit count is the same as rotating it right by the negated bit count: number.rotateLeft(-n) == number.rotateRight(n)
Rotating by a multiple of UInt128.SIZE_BITS (128) returns the same number, or more generally number.rotateLeft(n) == number.rotateLeft(n % 128)