Changed NonCamelCaseTypes lint to warn by default

Added allow(non_camel_case_types) to librustc where necesary

Tried to fix problems with non_camel_case_types outside rustc

fixed failing tests

Docs updated

Moved #[allow(non_camel_case_types)] a level higher.

markdown.rs reverted
auto merge of #12290 : mrshu/rust/lint-warn-by-default, r=alexcrichton

This first part of my attempts to fix #11432.

In this one I only set NonCamelCaseTypes to warn by default and tried to fix errors that were reported by `make check`.

Please feel free to let me know if I missed something or didn't do it the right way.

Thanks.