ed25519_dalek should be re-exported by the Rust SDK

[daweth]

ISSUE

ed25519_dalek is used for the SecretKey type but is not re-exported, making it impossible to construct a secretKey using the rust SDK alone.

As you can see here:

/// An Ed25519 private key
#[derive(DeserializeKey, SerializeKey, SilentDebug, SilentDisplay)]
pub struct Ed25519PrivateKey(pub(crate) ed25519_dalek::SecretKey);

The Ed25519 Private Key uses the SecretKey from ed25519_dalek crate

In the SecretKey Impl for ed25519_dalek: https://github.com/dalek-cryptography/ed25519-dalek/blob/main/src/secret.rs

    ///
    /// # Returns
    ///
    /// A `Result` whose okay value is an EdDSA `SecretKey` or whose error value
    /// is an `SignatureError` wrapping the internal error that occurred.
    #[inline]
    pub fn from_bytes(bytes: &[u8]) -> Result<SecretKey, SignatureError> {
        if bytes.len() != SECRET_KEY_LENGTH {
            return Err(InternalError::BytesLengthError {
                name: "SecretKey",
                length: SECRET_KEY_LENGTH,
            }.into());
        }
        let mut bits: [u8; 32] = [0u8; 32];
        bits.copy_from_slice(&bytes[..32]);

        Ok(SecretKey(bits))
    }

This function is required to create a new SecretKey from bytes since the Ed25519 library in the aptos-crypto crate does not have any constructors

https://github.com/aptos-labs/aptos-core/blob/main/crates/aptos-crypto/src/ed25519/ed25519_keys.rs

Therefore I believe that ed25519_dalek should be exported by aptos_crypto, or at lease relevant functions such as generate or from_bytes

[davidiw]

Could you submit a PR?

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