Add criterion benchmark to forc-doc + initial code quality pass

[JoshuaBatty]

Description

Doing some general code quality changes at the same time.

Checklist

• [ ] I have linked to any relevant issues.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [ ] I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • [ ] If my change requires substantial documentation changes, I have requested support from the DevRel team
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• [ ] I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• [ ] I have requested a review from the relevant team or maintainers.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #7242 will improve performances by 13.06%

_{Comparing josh/forc_doc (7e4be66) with master (0ac606c)}

Summary

⚡ 1 improvements
✅ 23 untouched benchmarks
🆕 1 new benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	build_std_lib_docs	N/A	299.1 ms	N/A
⚡	tokens_for_program	593.7 µs	525.1 µs	+13.06%