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
Update tasks guide: SharedChan as been removed

The code examples are up to date, but the surrounding explanations are not.
Added more scripts to tidy check
backticks for syntax elements
Spellcheck library docs.
Updated metadata::creader::resolve_crate_deps to use the correct span. Clarified error message when an external crate's dependency is missing. Closes #2404.
auto merge of #12146 : gentlefolk/rust/issue-2404, r=alexcrichton

Addresses FIXME described in issue #2404
auto merge of #12232 : kballard/rust/taskbuilder-is-a-builder, r=alexcrichton

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.

Closes #6399.
auto merge of #12243 : dguenther/rust/extend-tidy, r=alexcrichton

Extends the license and formatting check to `*.js` files in `src/doc` and `*.sh`, `*.pl`, `*.c`, and `*.h` files in `src/etc`. As best as I could tell, these files should be covered under the Rust project license.

cc @brson: Do any other scripts need a license? I'd like to double-check that this PR closes #4534.
auto merge of #12103 : alexcrichton/rust/unix, r=brson

There's a few parts to this PR

* Implement unix pipes in libnative for unix platforms (thanks @Geal!)
* Implement named pipes in libnative for windows (terrible, terrible code)
* Remove `#[cfg(unix)]` from `mod unix` in `std::io::net`. This is a terrible name for what it is, but that's the topic of #12093.

The windows implementation was significantly more complicated than I thought it would be, but it seems to be passing all the tests. now.

Closes #11201
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)
auto merge of #12314 : huonw/rust/is_utf8_iter, r=kballard

See the commit messages for more details, but this makes `std::str::is_utf8` slightly faster and 100% non-`unsafe` and uses a similar thing to make the first scan of `from_utf8_lossy` 100% safe & faster.
auto merge of #12351 : kud1ing/rust/backticks, r=alexcrichton
Tweak how preference factors into linkage

The new methodology can be found in the re-worded comment, but the gist of it is
that -C prefer-dynamic doesn't turn off static linkage. The error messages
should also be a little more sane now.

Closes #12133