More idiomatic return types for metadata getters

[davidsemakula]

Summary

Closes - N/A

• [y] y/n | Does it introduce breaking changes?
• [n] y/n | Is it dependent on a specific version of cargo-contract or pallet-revive?

More idiomatic return types for some metadata getters that return references to types that are cheap to copy.

Description

Changes a few metadata getter assoc fns that return &T when T: Copy to return T instead

Checklist before requesting a review

• I have added an entry to CHANGELOG.md
• I have commented on my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules