Improve definition of the `Message` struct


```rust
/// A (hashed) message input to an ECDSA signature.
#[derive(Copy, Clone)]
pub struct Message([u8; constants::MESSAGE_SIZE]);
```

This is not only used in ECDSA Signature, It is also used into the signing of schnorr signature.

https://github.com/rust-bitcoin/rust-secp256k1/blob/7c8270a8506e31731e540fab7ee1abde1f48314e/src/schnorr.rs#L130
