3.1: BigIntImpl::limb_from_bytes and BigIntImpl::from_bytes functions are simply unused. I removed them.
3.2: limb_to_le_bits_toaltstack function considers the case when n_bits=1 and n_bits=0, suggested solution.
3.3, 3.5: double_allow_overflow functions now expects the reference in BigInt units as in other functions, also hardcoded "9" is fixed.
3.4: I added an assertion to BigIntImpl struct that makes sure N_LIMBS is at least 2.
3.6: make sure most significant 1 lies inside the limits.
3.7: incorrect powers of two for n<7, although we dont encounter this case, is fixed by applying the suggested.
3.8: Bigint mul is simply unused. I removed it.
3.9: applied the suıggested solution.
3.10: u4_add_tables optimization is implemented and necessary hardcoded table size counts are updated.