Use weak features for preserve_order

[sunshowers]

Currently, preserve_order causes the toml, serde_json and ron dependencies to be enabled. This can be avoided by using cargo's support for [weak features].

[weak features]: https://rust-lang.github.io/rfcs/3143-cargo-weak-namespaced-features.html

Weak features were stabilized in Rust 1.60 so there are no MSRV concerns.

I think this probably is a breaking change since it's possible some downstream users weren't enabling support explicitly.

[matthiasbeyer]

Unfortunately, CI fails because your commit needs signoff. Please:

git commit --amend --signoff && git push <your remote> weak --force

to fix this.

[sunshowers]

@matthiasbeyer done, thanks!