Make missing documentation linting more robust

Add some more cases for warning about missing documentation, and also add a test
to make sure it doesn't die in the future.
Allow doc(hidden) and --test to disable doc linting
Require documentation by default for libstd

Adds documentation for various things that I understand.
Adds #[allow(missing_doc)] for lots of things that I don't understand.
libextra: Require documentation by default