Description to ECDSA adaptor signature and DLEQ proof types

[raulcano]

Following the discussion I initiated here #214
I believe it will be positive to add a detailed description to what exactly are the types ecdsa_adaptor_signature and dleq_proof. For this, I also add a new convenience type scalar.

[raulcano]

Just a kind reminder to review this update. I believe it's important to have the types ecdsa_adaptor_signature and dleq_proof properly documented.

One caveat though, the description I have added to the Messaging.md follows the original spec of "65 bytes for an adaptor sig" and "97 bytes for a dleq proof".
However, the ECDSA-Adaptor.md file still describes the adaptor signature as R || R_a || s_a || proof, which is not consistent with the description in Messaging.md.

I would propose to slightly modify that bit to something like:

Set a to R || s_a || R_a || proof
Return a
The return value is an adaptor signature along with the proof.

Note that the new return value has:

• R || s_a = 65 bytes, which is what I have described as the ecdsa_adaptor_signature type in this PR.
• R_a || proof = 97 bytes, which is what I have described as the dleq_proof type in this PR.

Probably we should work out a formulation to ensure consistency but given this discussion I'm not sure if we would open a can of worms.