Improve interoperabilities

[cmpute]

• Add from_le_bytes and from_be_bytes for IBig, equivalent to num_bigint::BigInt::{from_signed_bytes_le, from_signed_bytes_be}
• Add from_digits and to_digits for IBig, somewhat equivalent to num_bigint::BigInt::{from_radix_le, from_radix_be}, but it support radix in a whole Word.