fix: ensure matching ECDH shared secret

[mdqst]

No description provided.

[jonasnick]

Hi @mdqst, I'm pretty sure that the current ECDH example code produces the same shared secret for both parties. This is tested in CI and I just double checked locally.

[jonasnick]

The default hash function in libsecp256k1 always hashed the compressed representation of the point. So you want to change libsecp's example to match your code?

Note that outputting the X-coordinate directly can lead to vulnerabilities because the resulting shared secret is easily distinguishable from uniform randomness (and potentially enables "Cheon's attack").

[jonasnick]

Now you're outputting the X-coordinates of both participants' public keys (and remove a bunch of documentation comments). Why?