feat: Unix domain sockets with serde transports

[kbknapp]

Picks up where #369 apparently stalled. Funny fact I didn't look at existing PRs when doing this, so we just happened to pick almost identical impls since it's basically just copying the tcp module.

All tests are passing locally, I've also run this through a local application I have using unix sockets and it appears to work as desired.

The unix sockets are behind the feature flag unix and in the unix module to keep modules similar to those in Tokio and std. They are also gated by the target OS being a unix flavor as well.

~There's one minor additional tag along commit of a UI test that was failing on master.~ (edit: looks like that may have been an artifact of me testing with nightly. It's been removed).

[tikue]

Thanks so much! I'm away this week, will hopefully review next week

[dignifiedquire]

@tikue any chance of a release? I would love to start using this.

[tikue]

Sorry for the delay! I'll get it released this week.

[tikue]

It is now released, thank you for your patience! 🙏

[dignifiedquire]

thank you!