Allow "expanding" and "running" code examples in guide.

[zohaad]

Suggestion: make code examples on rocket.rs compilable like in the "expandable view" for the rust book code examples.

[jebrosen]

This needs a bit more thought before anything could be implemented. There are a few major differences from the rust book:

• The Book uses play.rust-lang.org to run code, which doesn't include rocket. We would either need to get rocket into the rust playground, or to duplicate it to some extent.
• In order for most examples to be useful, an example request is also needed. I think this goes beyond what we can expect from play.rust-lang.org, again pointing to needing to duplicate it in some way.
• Many examples are partial examples - routes that are never mounted, request guards that are never invoked, etc. Sometimes there is a "full example" at the end that brings them all together, and probably only that one should be "playable".

The need to duplicate and/or extend functionality from the playground looks like the biggest issue: it's significantly more difficult to maintain than rocket.rs is, unless there are existing code playpens besides play.rust-lang.org that we could integrate somehow.

[chinoto]

It would probably be too demanding, but I wonder if something in browser could be accomplished like WordPress is doing. Supposedly miri would be easier to compile to wasm that rustc: https://github.com/rust-lang/miri/issues/722