enable #[no_std] use by providing std feature

[Felix-El]

Also adds a dependency on no_std_net for no_std addresses.

#133

[mrmonday]

This looks great - thanks!

It would be good to have nostd tested in CI so this doesn't regress, but that can be done in another PR later on.

[fujiapple852]

@mrmonday @Felix-El this change has bumped the effective MSRV to 1.60 due to the use of the new dep-name?/feature-name Cargo syntax, specifically this change.

Was this intended?

[Felix-El]

I'm open to using a pre-1.60 Cargo syntax. I'm just not aware of it. Is there an alternative syntax for requesting a feature conditionally without forcing the dependency in?

[fujiapple852]

@Felix-El I'm not completely sure, the original GH RFC suggests "There exists a kind of workaround" which doesn't inspire confidence!

No issue with bumping the MSRV from time to time, it may be useful to add the conventional rust-version attribute to Cargo.toml and mention bumps prominently in release notes and the README to save people having to figure it out.

[mrmonday]

We used to support older Rust versions - we changed at some point to only support the latest stable Rust.

This said, I'm happy to accept a PR to make these changes place nicely with older Rust versions if that's possible.