Duplicate implementation of `calculate_ecdh_shared_secret()`

[pythcoiner]

calculate_ecdh_shared_secret() is implemented twice:

• in utils/sending.rs
• in utils/receiving.rs

it seems to me only docstrings & arguments names differs, maybe one should be just a wrapper of the other in order to drop the duplicate code?

[cygnet3]

Although these functionally do the same, the data that the caller provides represent different things. if I were to rewrite this, I would use custom structs like PartialSecret or TweakData to make that more explicit. We might do a big overhaul of this crate once bitcoin-core/secp256k1#1698 gets merged.