refactor: optimize bitset

[pscott]

We compute mask by using the pow method but we could use some bitwise operators there (need to check if index > 128 and do the bitshifting accordingly!)

sx-starknet/starknet/src/utils/bits.cairo

Line 16 in 687401f

let mask = pow(u256 { low: 2_u128, high: 0_u128 }, index);