Add support for NIST P-256 curve

[AlfioEmanueleFresta]

Changes

• Adds support for NIST P-256, aka secp256r1, using the p256 crate from RustCrypto. This is a pure-Rust implementation.
• As per feedback in previous abandoned PR try to add secp256k1_support #38 (comment), the code is feature gated, as this is a non-standard curve.
• Includes PR Update Cargo.toml #181 which updates curve25519-dalek, which unbreaks the build for me.

Motivation

This curve is used for the Noise handshake in CTAP2 Hybrid Transport. This is the protocol used for browsers to access FIDO2 credentials (aka Passkeys) on roaming authenticators connected over the internet, such as phones running Android and iOS.

I'm using snow in my implementation of CTAP2, aimed at bringing Passkeys to the Linux desktop, see xdg-credentials-portal.

Tests

All commands below succeed:

$ cargo build --all-targets
$ cargo build --all-targets --features p256
$ cargo test --all-targets
$ cargo test --all-targets --features p256