Use `BigUint` for the Signature internal representations

[lumag]

Ideally we could replace the internal representation of a Signatures with a BigUint now, which would eliminate the need for an intermediate Box<[u8]> when signing/verifying.

Originally posted by @tarcieri in #217 (comment)

[tarcieri]

cc @npmccallum

There are many APIs that currently take a slice which would need at least an equivalent API which operates on a BigUint.

I think we could potentially consider removing some of the old APIs from the public API as well.

[tarcieri]

Implemented in #298 and #300