Move MSRV test to separate job with cargo +nightly update -Z minimal-versions

[ilslv]

This PR moves MSRV test into a separate job and adds cargo +nightly update -Z minimal-versions to downgrade into lowest compatible version from Cargo.toml.

For now, this crate depends on syn v1.0.0 and higher, which isn't quite true, because Path::get_ident() method seen here wasn't introduced until syn v1.0.3. To avoid problems like this, I've fixed main dependencies version to latest stable ones.

[tyranron]

Run CI

[ilslv]

@tyranron bumping rustc_version to 0.4.0 fixes MSRV locally for me

[tyranron]

@ilslv so, at this moment -Z minimal-versions doesn't work because of peg 0.5 in [build-dependencies] not working correctly with -Z minimal-versions.

Actually there is a peg 0.7 already, but I doubt @JelteF will agree migrate to it, because at the time the whole idea of introducing peg was to avoid additional [dependencies] for parsing, while starting from peg 0.6 it works like [dependencies] not the [build-dependencies].

Is there any alternatives to peg which can produce parser in build time without living in direct [dependencies], while are maintained?

[tyranron]

@ilslv

bumping rustc_version to 0.4.0 fixes MSRV locally for me

Oh, nice! ♥️

So we can postpone peg question for now.

[tyranron]

@ilslv now it doesn't work because 1.36 Rust doesn't understand well newer .lock file format. 🙈

I guess we need for now to separate check for -Z minimal-versions and MSRV in different jobs, until MSRV is bumped up.

[tyranron]

@JelteF to summaize this PR:

• adds CI job checking whether crate works OK on minimal crates versions described in Cargo.toml
• fixes Clippy lints on latest nightly

[tyranron]

@JelteF can we hope for a patch release with this, so downstream crates won't fail on -Z minimal-versions check?

[JelteF]

I released v0.99.17 just now, which includes this change.