Switch to prettyplease formater

[rpolishchuk]

Not everyone is living on the edge (see #843) :)

This commit switches to prettyplease (noted as an option in #843 (comment)) fix test failures with the stable toolchain similar to:

called `Result::unwrap()` on an `Err` value: Unstable("wrap_comments, normalize_doc_attributes, format_strings")

Also split_multiline_doc_attributes preprocess comments to avoid switch from /// to /** */

[ahl]

I'm not sure that this is the right approach for cargo-progenitor in particular. It emits a stand-alone crate--I think there's a reasonable expectation for that crate to be rustfmt compliant. I do love prettyplease, but its output isn't the same (as evidenced by the 45k lines of change in this PR.

[ahl]

Happy to discuss this, but if I don't hear from you, I'll close this next week; we can always re-open