update ink! attribute argument syntax docs and simplify ink! attribute meta parsing implementation in ink_ir

[davidsemakula]

Summary

Closes #_

• [n] y/n | Does it introduce breaking changes?
• [n] y/n | Is it dependant on the specific version of cargo-contract or pallet-contracts?

Description

• Updates ink! attribute argument syntax documentation in ink_ir's ast module to accurately describe current deviations from Rust "meta item" attribute syntax.
• Simplifies ink! attribute meta parsing implementation in ink_ir's ast::meta module by removing unnecessary custom utilities while maintaining support for parsing the described ink! attribute argument syntax.

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have added an entry to CHANGELOG.md
• I have commented 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

[cmichi]

Sorry for not providing feedback earlier, @davidsemakula. Is that why you closed the PR?

[davidsemakula]

Hi @cmichi , no problem ... I just decided that it was better to include changes from this that are still relevant in more recent/new PRs 🙂