Package uint128 provides a basic unsigned 128 bit integer implementation.
Add sets `a` to the full 128 bit result of `a + b`.
Add64 sets `a` to the full 128 bit result of `a + b`.
Hi returns the high 64 bits of `a`.
Lo returns the low 64 bits of `a`.
Mul sets `out` to the full 128 bit result of `a * b`.
Shl returns the high 64 bits of `a << shift`.
Shr returns the low 64 bits of `a >> shift`.
Uint128 is an unsigned 128 bit integer represented as 2 uint64 limbs.