Replace peg crate with manually implemented parser

[ilslv]

In 0.6 release of peg build.rs parser generation support was removed, so to keep our dependencies up to date, we've decided to re-implement parser. Design is heavily inspired by nom crate.

This PR also fixes some inconsistencies with original format_string syntax and adds support for non-ascii identifiers. To make this possible I've added 1 dependency: unicode-xid, but actually syn crate already depends on it.

Also parser supports non-integer parameters, because in follow-up PR I'm planning to add support for interpolated strings introduces in 1.58 release while preserving MSRV.

[tyranron]

@ilslv LGTM, thanks! 🍻

@JelteF I'd like you to look at this as well, whether you're OK with these changes.

[tyranron]

ping @JelteF

[JelteF]

I haven't looked in detail at the new parser code, but it looks much more understandable than the previous auto generated one. Given none of our display tests fail, I think it's fine to merge.