fn shiftLeft(r: *Mutable, a: Const, shift: usize) void
r = a << shift, in other words, r = a * 2^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,