Add U256/U384 and mod-const-prime wrapper utilities of both.

Add U256/U384 and mod-const-prime wrapper utilities of both.

In the next commit we'll add support for secp256r1 and secp384r1
validation, which require 256-bit and 384-bit integers. To make
their implementation simple, we also add wrapper structs around
the new integers which are modulo a const-prime, storing and
handling the values in montgommery representation.