Remove Real trait and move methods into Float

This is part of the effort to simplify `std::num`, as tracked in issue #10387.
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!
Remove the compile-fail test that's now obsolete
backticks for syntax elements
Spellcheck library docs.
Fix a segfault in the rustuv tests

The details can be found in the comments I added to the test, but the gist of it
is that capturing output injects rescheduling a green task on failure, which
wasn't desired for the test in question.

cc #12340
Test ordering of glue_drop and glue_takew in self-re-assignment. Closes #3290.
Made fail_bounds_check more careful with strings.

Fixes GH #11976.
Update compiler-rt

Closes #12356
auto merge of #12321 : bjz/rust/remove-real, r=alexcrichton

This is part of the effort to simplify `std::num`, as tracked in issue #10387. It is also a step towards a proper IEEE-754 trait (see #12281).
auto merge of #12342 : ehsanul/rust/remove-shared-chan-tasks-guide, r=alexcrichton

The code examples are up to date, but the surrounding explanations are not.
auto merge of #12345 : huonw/rust/speeling, r=cmr
std::str: safen and optimize is_utf8.

This uses a vector iterator to avoid the necessity for unsafe indexing,
and makes this function slightly faster. Unfortunately #11751 means that
the iterator comes with repeated `null` checks which means the
pure-ASCII case still has room for significant improvement (and the
other cases too, but it's most significant for just ASCII).

Before:

    is_utf8_100_ascii             ... bench:       143 ns/iter (+/- 6)
    is_utf8_100_multibyte         ... bench:       134 ns/iter (+/- 4)

After:

    is_utf8_100_ascii             ... bench:       123 ns/iter (+/- 4)
    is_utf8_100_multibyte         ... bench:       115 ns/iter (+/- 5)
std: convert first_non_utf8_byte to use the iterator.

This makes it very slightly faster, especially when the string is valid
UTF-8, and completely removes the use of `unsafe` from the first half.

Before:

    from_utf8_lossy_100_ascii              ... bench:       151 ns/iter (+/- 17)
    from_utf8_lossy_100_invalid            ... bench:       447 ns/iter (+/- 33)
    from_utf8_lossy_100_multibyte          ... bench:       135 ns/iter (+/- 4)
    from_utf8_lossy_invalid                ... bench:       124 ns/iter (+/- 10

After:

    from_utf8_lossy_100_ascii              ... bench:       119 ns/iter (+/- 8)
    from_utf8_lossy_100_invalid            ... bench:       454 ns/iter (+/- 16)
    from_utf8_lossy_100_multibyte          ... bench:       116 ns/iter (+/- 9)
    from_utf8_lossy_invalid                ... bench:       119 ns/iter (+/- 9)