Make it easier to compile examples from the tutorial

[catamorphism]

On IRC, bbrittain pointed out that the map example in Section 8.1 (Generic Functions) doesn't compile on its own, because it needs an import statement in order to get the impl that provides each. To make the tutorial more usable, perhaps examples that need extra imports to compile and run could be discreetly marked... or they could include the imports, but in smaller, light gray text or something.

[catamorphism]

The particular example here does seem to compile on its own with Rust HEAD, so it's possible bbrittain was using an older version... but still, language near the beginning of the tutorial suggests that some snippets won't compile on their own, and in general, it would still be good to mark them.

[brson]

Examples in the docs may prefix lines with special comments (## maybe) to add unprinted test lines. That's still not enough to run some of the xfailed doc tests.

[jruderman]

Related to #1593

[metajack]

All the other related bugs have been closed. Is this still an issue?

[msullivan]

It looks like a lot of this is fixed?

[pnkfelix]

Accepted for P-low.

[steveklabnik]

I actually like linking these to the sandbox, as a solution.

[alexcrichton]

We actually do this by default now (linking to play.rust-lang.org), so closing.