feat: add DTLS 1.2 ChaCha20 and X25519 support

[HMBSbige]

Closes #71

Add TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 (0xCCA9) cipher suite and X25519 key exchange support for DTLS 1.2.

Simplify dependency version specs — Cargo.lock pins exact versions, so Cargo.toml only needs to declare the semver-compatible range.

This PR depends on the other open PRs (#73, #75, #76). Please merge those first before reviewing this one. Thanks!

[algesten]

Hey, I think this needs changing right?

• Iv::as_12_bytes() uses debug_assert_eq for a crypto invariant

  pub(crate) fn as_12_bytes(&self) -> &[u8; 12] {
      debug_assert_eq!(self.len(), 12, ...);
      &self.bytes
  }

If this is accidentally called on a 4-byte AES-GCM IV in release builds, the remaining 8 bytes are zeros — potential
nonce reuse. Should be a regular assert_eq!.

---

[HMBSbige]

Fixed!