Use big-endian in all of the nameaccumulator spec.

[matheus23]

This is to avoid errors in implementation.
Some of the reasoning copied from this comment: #66 (comment)

• Having both big & little endianess in the same protocol sucks. rs-wnfs had the endianess of some things wrong now for a while without me noticing. That's bad.
• Most protocols that deal with RSA are big-endian. And because e.g. we're importing RSA moduli from the outside sometimes, we should also be big-endian. More concretely: I don't want an implementation to import a WebCrypto-generated RSA modulus (which is big-endian) and rs-wnfs interpreting that as little-endian and suddenly it's not a safe RSA modulus at all!
• The performance difference between little & big endianess was in fact so minuscule, I didn't notice I used the wrong endianess for a while.