Fix repeated module documentation
Refactor ty_str to use a ~(str) representation.

Similar to my recent changes to ~[T]/&[T], these changes remove the vstore abstraction and represent str types as ~(str) and &(str). The Option<uint> in ty_str is the length of the string, None if the string is dynamically sized.
rustdoc: Make strip_hidden use a dedicated hidden item if any.

fixes #13806.
auto merge of #13812 : alxgnon/rust/master, r=alexcrichton

This is a quick fix for repeated documentation descriptions in certain modules.
Following is a list of the faulty modules I found. I ran `pcregrep -r -M "<p>(.+)\n\1" doc` on the html documentation to help identify them.

- [rustuv::uvio](http://static.rust-lang.org/doc/master/rustuv/uvio/index.html)
- [rustuv::uvll](http://static.rust-lang.org/doc/master/rustuv/uvll/index.html)
- [std::rt::backtrace](http://static.rust-lang.org/doc/master/std/rt/backtrace/index.html)
- [std::rt::env](http://static.rust-lang.org/doc/master/std/rt/env/index.html)
- [std::rt::global_heap](http://static.rust-lang.org/doc/master/std/rt/global_heap/index.html)
- [std::rt::local_heap](http://static.rust-lang.org/doc/master/std/rt/local_heap/index.html)
- [std::rt::rtio](http://static.rust-lang.org/doc/master/std/rt/rtio/index.html)
- [std::rt::task](http://static.rust-lang.org/doc/master/std/rt/task/index.html)
- [std::rt::thread](http://static.rust-lang.org/doc/master/std/rt/thread/index.html)
- [std::rt::unwind](http://static.rust-lang.org/doc/master/std/rt/unwind/index.html)
- [syntax::parse::classify](http://static.rust-lang.org/doc/master/syntax/parse/classify/index.html)
- [syntax::parse::common](http://static.rust-lang.org/doc/master/syntax/parse/common/index.html)

After a little testing, I discovered that moving the documentation inside (`//!`) instead of outside (`///`) modules fixed the immediate problem. I went through the trouble of moving the documentation, and with this commit there are no more repeated descriptions within those faulty modules.

This does not fix the underlying problem though. We should look into why having the documentation outside instead of inside caused the descriptions to be repeated. I will create a separate issue with my findings on the subject if necessary.
In the meantime, this simple fix should be enough.
auto merge of #13819 : nick29581/rust/ty_str, r=pcwalton

Similar to my recent changes to ~[T]/&[T], these changes remove the vstore abstraction and represent str types as ~(str) and &(str). The Option<uint> in ty_str is the length of the string, None if the string is dynamically sized.
auto merge of #13827 : lifthrasiir/rust/rustdoc-hidden-pub-field, r=alexcrichton

Fixes #13806. Also adds a note to `HiddenStructField` about why it doesn't appear in the `clean` module itself.