Add doc examples for most untrusted::Input methods.

[frewsxcv]

I agree to license my contributions to each file under the terms given
at the top of each file I changed.

[frewsxcv]

I like documentation when they include examples and so does this RFC. @briansmith No feelings hurt if you feel otherwise. I couldn't think of a great example for read_all_mut and I'm a bit short on time so I didn't include one there. Let me know your thoughts and ideas for improvement.

[briansmith]

Thanks for this.

First, I think that your examples would be great to have as minimal unit tests. So, I suggest moving them to tests instead.

One of the problems that untrusted::Input tries to solve is DoS caused by panics. So, while I totally understand what you're trying to do here, I don't think that these code samples are actually good models for people to follow, because assert!() and assert_eq!() panic. Do you see what I mean here? (Sorry it's probably not very clear.)

So, instead, I think it would be better for the documentation to do similar stuff, but without using panicing stuff. e.g.

if a.is_empty() {
    return Err(());
}

Again, thanks for submitting this.

[frewsxcv]

Extracted out the examples into unit tests: #4

[frewsxcv]

I got rid of the panics. Not sure what you'll think of the changes. I'm pretty neutral.

[briansmith]

Sorry I never got around to looking at this. I'm just going to close this now to clear the backlog. I do think we should have some better documentation but I think we should refactor the API first.

[briansmith]

Also, thanks for the PR!