Move regex-capi to 2021 & inline format-args

[nyurik]

This makes the code a bit easier to read and smaller.

The capi crate was updated to the 2021 edition to avoid accidental panic! and other edge cases with inlined formats.

Some of it was done with this command, and later fixed by hand:

 cargo clippy --workspace --allow-dirty --fix --benches --tests --bins -- -A clippy::all -W clippy::uninlined_format_args

[nyurik]

fixed

[nyurik]

@BurntSushi hi, friendly ping - let me know if anything else is needed for this and other regex PRs. Thx!