Document for-loop in tutorial section on loops
 Explain container iteration in the loop tutorial

 As suggested by @pnkfelix in #12161, this extends the examples by a for-loop
 that iterates over a string as an example for container iteration.
auto merge of #12161 : aepsil0n/rust/docs/for-loop, r=alexcrichton

I just started learning Rust and the absence of an explanation of the for-loop in the beginning really bugged me about the tutorial. Hence I simply added these lines, where I would have expected them. I know that there is something later on in the section on traits. However, this simple iteration scheme feels like something that you should be aware of right away.
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

Fixed timer that was failing tests