Consider exposing field in the public API

[cjpatton]

I have in mind an application that uses GF(2^255-19) for secret sharing rather than elliptic curves: https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-vdaf-03. We picked this field because of its large size (~256 bits) and to take advantage of high-quality implementations already available.

This crate seems to have the best implementation for rust. Would you consider adding the module to the public API? Do you foresee any technical difficulties with doing so?