fn shiftRight(r: *Mutable, a: Const, shift: usize) void
r = a >> shift r and a may alias.
Asserts there is enough memory to fit the result. The upper bound Limb count is a.limbs.len - (shift / (@sizeOf(Limb) * 8)).
a.limbs.len - (shift / (@sizeOf(Limb) * 8))
r: *Mutable,
a: Const,
shift: usize,