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.
Adds support for working with URL Paths

It is sometimes useful to parse just the path portion of a URL (path,
query string and fragment) rather than the entire URL.
Remove Real trait and move methods into Float

This is part of the effort to simplify `std::num`, as tracked in issue #10387.
Clean up std::task docs, make TaskBuilder a real builder

Delete all the documentation from std::task that references linked
failure.

Tweak TaskBuilder to be more builder-like. .name() is now .named() and
.add_wrapper() is now .with_wrapper(). Remove .watched() and
.unwatched() as they didn't actually do anything.
Update clients of the TaskBuilder API
Implement Unix domain sockets in libnative
Move unix pipes implementation to pipe_unix.rs

The windows named pipes implementation will have almost nothing to do with unix
pipes, so I think it's best if they live in separate files.
Implement named pipes for windows, touch up unix

* Implementation of pipe_win32 filled out for libnative
* Reorganize pipes to be clone-able
* Fix a few file descriptor leaks on error
* Factor out some common code into shared functions
* Make use of the if_ok!() macro for less indentation

Closes #11201
Remove Freeze bounds from sync::MutexArc

With Rc no longer trying to statically prevent cycles (and thus no
longer using the Freeze bound), it seems appropriate to remove that
restriction from MutexArc as well.
Update tasks guide: SharedChan as been removed

The code examples are up to date, but the surrounding explanations are not.
std: iteratize str::is_utf16 & add tests.

Most of the tests are randomly generated with Python 3 and rely on it's
UTF-16be encoder/decoder being correct.
std: convert str::from_utf16 to an external iterator.

Fixes #12316.
str: provide lossy UTF-16 support.

This replaces the iterator with one that handles lone surrogates
gracefully and uses that to implement `from_utf16_lossy` which replaces
invalid `u16`s with U+FFFD.
std: decode even numbered non-BMP planes in the UTF-16 decoder.

Fixes #12318.
std: make str::from_utf16 return an Option.

The rest of the codebase is moving toward avoiding `fail!` so we do it
here too!
Added more scripts to tidy check
Remove the compile-fail test that's now obsolete
backticks for syntax elements
Spellcheck library docs.