Warnings caused by examples in libstd should fail the doc tests

[crumblingstatue]

Recently, an incorrect example was discovered in the libstd documentation.

In this case, the compiler gives a warning for unused results on the I/O operations.
The example should have checked these results, uncovering a bug, which is the file being opened in read-only mode when it was intended to be written to.

If warnings would fail the doc tests, this issue would have been caught automatically.

In case an example wants to explicitly allow "questionable" constructs, it can use the allow(lint) attribute.

[sfackler]

We definitely want to allow some warnings, like dead_code, but it seems like adding a default #[deny(unused_result)] would be a good idea.

[crumblingstatue]

How about allowing dead_code (and perhaps some other things that are commonly valid for examples), and denying everything else by default?

[steveklabnik]

The reason warnings weren't happening is that they were actually suppressed.