chore: explicit IO methods in interfaces

[ivokub]

Description

Previously the Groth16 interfaces referred to groth16Object. As it wasn't exported, then it wasn't obvious that the interfaces implement the IO methods. I made the methods now explicit for better reference.

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules