Updating variable name from sk_iss and idk to imk

[vivek-arte]

This PR updates the notation for the issuance keys in order to make it consistent with the spec.

[what-the-diff]

PR Summary

This Pull Request (PR) involves a significant update of the terminology used in several files of the codebase. The term sk_iss has been replaced with imk throughout these files, which helps to increase code clarity and consistency. Here are the key changes:

• Refactoring in issuance.rs file

  • The term sk_iss has been updated to imk on lines 626 and 1276 for better variable representation.

• Updates in keys.rs file

  • Renamed sk_iss to imk on lines 259, 314 and 1269 improving the readability and understanding of the code.

• Renaming in note/asset_base.rs file

  • The term sk_iss has been replaced with imk on lines 104 and 169 enhancing consistency across codebase.

• Changes in supply_info.rs file

  • Line 84 saw the replacement of sk_iss with imk which offers better variable naming conventions.

• Adjustments in tests/zsa.rs file

  • The occurrence of sk_iss on line 63 is now changed to imk, streamlining code terminology.

These changes do not modify the functionality of the code, but they significantly improve the readability and consistency of the codebase. This makes future development and maintenance easier.

[vivek-arte]

Closed in favour of #92