Implement utility functions for the new Bignum types

[yanesca]

Prerequisites: #6017

Add the following functions to bignum_mod and bignum_mod_raw

• _copy(): Takes two MPIs and a modulus as parameters and copies one to the other
• _set_zero(): Takes an MPI as an argument and sets it to zero
• _set_one(): Takes an MPI and a modulus as an argument and sets it to zero in the appropriate representation
• _length(): Takes an MPI as an argument and returns the length of the canonical representative (_mod version will need conversions) in bytes (without leading zeroes)