Updates to Schnorr, ECDH, and Context

[csjones]

⬆️

[what-the-diff]

PR Summary

• Version update in README
  The version of the secp256k1.swift package cited in the README.md file has been nudged up from 0.12.0 to 0.13.0.

• New File Additions
  Two new files titled Context.swift have been added - one each in the secp256k1 and the zkp folders.

• Revisions in README's Schnorr Section
  Notes have been appended in the Schnorr section of the README.md: firstly to highlight that strict BIP340 mode is now turned off by default for Schnorr signatures tackling variable length messages, and secondly to state that the sharedSecret can now be formed in a more compact format.

• Update in Silent Payments Scheme Section
  The Silent Payments Scheme portion of the README.md file has been updated with the revised process of forming a shared secret using only a public key - now resulting in a more condensed shared secret.

• Multiples Code Enhancements
  Several tweaks have been applied across various files that primarily aim at increasing code stability and compactness, and improving the interpretation of byte details in the secp256k1.swift. The type of signature format that the application uses can also be specified more easily now.

• Removal and Test Function Additions
  The create function, previously associated with the secp256k1.Context extension, has been eliminated. New test functions have been assembled under secp256k1Tests to check new features and validate old ones after adjustments.