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!
Made fail_bounds_check more careful with strings.

Fixes GH #11976.
Update compiler-rt

Closes #12356
mk: Fix the regexp of SHOW_DOCS

The tag marks were missing, and `make tips` didn't work.
mk: Talk about `make clean` in the build help
Removing '15.3 Do syntax' in tutorial.

The 'do' keyword was deprecated in 0.10 #11868 , and is keep as
reserved keyword #12157 .

So the tutorial part about it doesn't make sense.
The spawning explanation was move into '15.2 Closure compatibility'.

Fixing misspelling.

Thanks for precisions.

Moved from 15.2 to 15.1.

Fixed typo, and apply pnkfelix advices.
Avoid returning original macro if expansion fails.

Closes #11692. Instead of returning the original expression, a dummy expression
(with identical span) is returned. This prevents infinite loops of failed
expansions as well as odd double error messages in certain situations.
auto merge of #12357 : chromatic/rust/gh_11976_fail_bounds_check_str, r=alexcrichton

Fixes #11976.
Clarify unit of size in docs for size_of functions.
Changed the docs in mem.rs to clarify the fact the the size functions return sizes in bytes.
Regression test closes #7660
auto merge of #12359 : alexcrichton/rust/update-compiler-rt, r=luqmana

Closes #12356
Fix staticlib outputs linking to blank archives

When creating a staticlib, it unzips all static archives it finds and then
inserts the files manually into the output file. This process is done through
`ar`, and `ar` doesn't like if you specify you want to add files and you don't
give it any files.

This case arose whenever you linked to an archive that didn't have any contents
or all of the contents were filtered out. This just involved ignoring the case
where the number of inputs we have is 0, because we don't have any files to add
anyway.
Fix bug with zero-length filemaps and rename bytepos_to_local_charpos to bytepos_to_charpos.
Make priavcy checking aware that a `use` directive can point to two defintions (namespaces) with different privacy. Closes #4110
auto merge of #12245 : nick29581/rust/priv2, r=alexcrichton

closes #4110
str: add a function for truncating a vector of u16 at NUL.

Many of the functions interacting with Windows APIs allocate a vector of
0's and do not retrieve a length directly from the API call, and so need
to be sure to remove the unmodified junk at the end of the vector.