Simplify CI, fix cargo test, and fix docstrings

[simlay]

hey hey, saw some of the cool stuff with objc2 coming into this repo so I figured I'd throw in some help that's independent of the objc2 changes.

This PR updates:

• CI to use cargo test which builds all the examples and their rust in docstrings.
• Updates as many docstrings as I could to be "correct". I don't know if all the semantics of the examples make a ton of sense but I did my best.
• Puts #[cfg(feature = "foo")] for examples that require feature foo (which are used in uikit and webview).

I figure this will enable us to have tests and prevent things like #32 are less likely to happen.

[madsmtm]

Wonderful, been wanting to do this as well!

[ryanmcgrath]

Huh, thought I responded to this via email.

Super appreciate this! It all looks good to me and I think just needs a formatting pass to satisfy the PR checks here. Happy to merge after that. :)

[simlay]

It all looks good to me and I think just needs a formatting pass to satisfy the PR checks here.

hmm. When I run cargo fmt locally, it changes a lot. This could be from the recent rust release or maybe there's something not obvious?

[ryanmcgrath]

shrug

Beats me - but it passes it now and that's good enough for me. I might go ahead and cut a new point release for this just so anyone looking at docs has your work here visible - and then I think 0.4 can be a target for objc2 PRs.

Seriously appreciate this PR!