ecdsa: Elliptic Curve Digital Signature Algorithm support

This module implements the elliptic curve digital signature algorithm for
a selection of curves supported by [[crypto::ec]].

This is a low-level module which implements cryptographic primitives. Direct
use of cryptographic primitives is not recommended for non-experts, as
incorrect use of these primitives can easily lead to the introduction of
security vulnerabilities. Non-experts are advised to use the high-level
operations available in the top-level [[crypto::]] module.

Be advised that Hare's cryptography implementations have not been audited.