determine MSRV a priori, lint, improve README

[peku33]

1 of 2 pull requests for adding portable-atomic support.

This focuses on some lints / fixes / improvements I wanted to make before publishing actual feature:

• determined MSRV a priori to be 1.60 (won't build on 1.59 because of feature/dep syntax) and put in the Cargo.toml
• updated edition to 2021 (compatible with 1.60)
• fixed or silenced clippy warnings
• reformatted with rustfmt
• improved README.md
• verified nothing breaks and MSRV is still 1.60