collections: Examples for as_string and as_vec

[bluss]

Improve example for as_string and add example for as_vec

Provide a better example of as_string / DerefString's unique capabilities.
Use an example where (for an unspecified reason) you need a &String, and
show how as_string solves the problem without needing an allocation.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[bluss]

Last time this came up I didn't communicate clearly enough why this function needed a (seemingly) silly example #19628. Don't "fix" it to lose the point of this peculiar function this time. 😄

[steveklabnik]

&String should be in `s

[alexcrichton]

@bors: r+ 84ef3b5 rollup

Thanks!