rustdoc: Add the ability to test code in comments

This adds support for the `--test` flag to rustdoc which will parse a crate,
extract all code examples in doc comments, and then run each test in the
extra::test driver.
std: Fix all code examples
extra: Fix all code examples
mk: Run doc tests as part of 'make check'

Don't run doc tests during make check-fast because it involves spawning lots of
processes.
doc: Expand rustdoc's documentation for testing