Function num_biguint_from_u64_limbs 

pub fn num_biguint_from_u64_limbs<I>(limbs: I) -> BigUint
where
    I: IntoIterator,
    I::Item: Borrow<u64>,

Builds a [num_bigint::BigUint] from little-endian u64 limbs.

The limbs are little-endian and each limb is itself little-endian, so their concatenated little-endian bytes are the value’s little-endian byte string.