Upgrade to Rust 2021

[TimTheBig]

Move to Rust 2021 and Refactor

Updates

Rust edition 2018 –> 2021
Version 2.5.2 –> 2.6.0
msrv according to cargo-msrv 1.40.0 –> 1.60.0, tested on x64 and arm64.

Small changes

Added "xdg" as keyword in cargo.toml
Add Swatinem/rust-cache to ci.yml and docs.yml for CI speed up.
Add CI, Doc-test, msrv and version badges to README.md.
actions/checkout v3 –> v4
Make lint workflow continue on fmt error, so clippy can also run.

[whitequark]

Thanks! It looks like clippy is still unhappy: https://github.com/TimTheBig/rust-xdg/actions/runs/9257836687/job/25466781313#step:6:22

[TimTheBig]

The main clippy complaint does make the code a lot cleaner so I would consider it:

self.state_home.as_ref().map(|home| home.as_path()),
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using`self.state_home.as_deref()`

I can also make clippy ignore it with #[allow(clippy::option_as_ref_deref)] if you prefer.

[whitequark]

Yeah, let's apply that.

[whitequark]

Thanks!