rustdoc: clean up generic impls

r? @QuietMisdreavus

Cleanup for librustc::session

Some rather straightforward cleanup. Each commit mostly stands alone.

Rollup of 14 pull requests

Successful merges:

 - #51919 (Provide `{to,from}_{ne,le,be}_bytes` functions on integers)
 - #52940 (Align 6-week cycle check with beta promotion instead of stable release.)
 - #52968 (App-lint-cability)
 - #52969 (rustbuild: fix local_rebuild)
 - #52995 (Remove unnecessary local in await! generator)
 - #52996 (RELEASES.md: fix the `hash_map::Entry::or_default` link)
 - #53001 (privacy: Fix an ICE in `path_is_private_type`)
 - #53003 (Stabilize --color and --error-format options in rustdoc)
 - #53022 (volatile operations docs: clarify that this does not help wrt. concurrency)
 - #53024 (Specify reentrancy gurantees of `Once::call_once`)
 - #53041 (Fix invalid code css rule)
 - #53047 (Make entire row of doc search results clickable)
 - #53050 (Make left column of rustdoc search results narrower)
 - #53062 (Remove redundant field names in structs)

Remove redundant field names in structs

This is a clippy-breaking change.

Make left column of rustdoc search results narrower

To make more room for the description of the item

The description often has useful text that helps disambiguate between search results, but very little of it is shown.

As a side effect, this breaks the alignment between the search results and the "In Return Types" tab, which tends to line up above the description-- up until I started investigating this, I thought "In Names"/"In Parameters"/"In Return Types" were column headers and I just never saw search results that had info for the "In Parameters" middle column! Now, with the two columns of search results each taking up about a half, they look more like tabs than column headers.

Types that are long still wrap and look good-- I made some artificially long types in the following screenshots.

Before screenshot:

<img width="1258" alt="screen shot 2018-08-03 at 8 32 35 pm" src="https://user-images.githubusercontent.com/193874/43670805-56e3b3b4-975e-11e8-9296-600837d03de2.png">

After screenshot:

<img width="1239" alt="screen shot 2018-08-03 at 8 31 17 pm" src="https://user-images.githubusercontent.com/193874/43670810-6591f9ac-975e-11e8-9e12-4ea9ab1e5806.png">

Provide `{to,from}_{ne,le,be}_bytes` functions on integers

If one doesn't view integers as containers of bytes, converting them to
bytes necessarily needs the specfication of encoding.

I think Rust is a language that wants to be explicit. The `to_bytes`
function is basically the opposite of that – it converts an integer into
the native byte representation, but there's no mention (in the function
name) of it being very much platform dependent. Therefore, I think it
would be better to replace that method by three methods, the explicit
`to_ne_bytes` ("native endian") which does the same thing and
`to_{le,be}_bytes` which return the little- resp. big-endian encoding.

The old issue has already been in FCP, a new issue was opened for the
new API.

Make entire row of doc search results clickable

By adding empty `after` content that clears and is `display: block`.
Technique found here: https://stackoverflow.com/a/7817313/51683

Now any part of a documentation search result that is highlighted when
you hover over it should also be clickable.

NOTE: THE BELOW IMAGES ARE GIFS BUT I DIDN'T MAKE THEM LOOP, OPEN IN A NEW TAB TO SEE THEM SORRY SORRY

Here's what happens before this change: my mouse cursor is in the middle of the table cell, and the row is highlighted which makes it look like clicking should have an effect, but clicking doesn't do anything:

![rustdoc-clicky-clicky-before](https://user-images.githubusercontent.com/193874/43668604-d1e63392-974b-11e8-99f5-e7e11c9056e8.gif)

After this change, clicking in the middle of the cell works!

![rustdoc-clicky-clicky-after](https://user-images.githubusercontent.com/193874/43668616-e522fa08-974b-11e8-8409-3826d586d1f7.gif)

It even works nicely when the type is really long and wraps! (I created the type artificially because I couldn't find a long enough one in the wild)

![rustdoc-clicky-clicky-after-long](https://user-images.githubusercontent.com/193874/43668639-f962a75c-974b-11e8-9218-70bf068387a3.gif)

Fix invalid code css rule

Fixes #53017.

r? @QuietMisdreavus

Specify reentrancy gurantees of `Once::call_once`

I don't think the docs are clear about what happens in the following code

```rust
static INIT: Once = ONCE_INIT;

INIT.call_once(|| INIT.call_once(|| println!("huh?")));
```

[Playground](https://play.rust-lang.org/?gist=15dde1f68a6ede263c7250c36977eade&version=stable&mode=debug&edition=2015)

Let's "specify" the behavior to make it clear that the current behavior (deadlock I think?) is not a strict guarantee.

volatile operations docs: clarify that this does not help wrt. concurrency

Triggered by https://github.com/rust-lang/rust/pull/52391. Cc @stjepang @Amanieu

Should the intrinsics themselves also get more documentation? They generally do not seem to have much of that.

Stabilize --color and --error-format options in rustdoc

Fixes #52980

cc @kennytm

r? @QuietMisdreavus

privacy: Fix an ICE in `path_is_private_type`

Fixes https://github.com/rust-lang/rust/issues/52879

(Untested.)

RELEASES.md: fix the `hash_map::Entry::or_default` link

None

Remove unnecessary local in await! generator

This makes `async { await!(future::ready(())) }` 4 bytes instead of 16. [Playground example](http://play.rust-lang.org/?gist=54c075599b9ff390fe505c75d6b98feb&version=nightly&mode=debug&edition=2018)

rustbuild: fix local_rebuild

If we detect a local rebuild (e.g. bootstrap compiler is the same version as target compiler), we set stage to 1.
When trying to build e.g. UnstableBook, we use Mode::ToolBootstrap and stage is 1.
Just allow Mode::ToolBootstrap and stagge != 0 if we are in a local_rebuild

This fixes building current master using current beta (as master hasn't yet been bumped to 1.30).
This should be backported to beta too, as currently we cannot build beta using itself because of that.

r? @alexcrichton

App-lint-cability

@eminence recently pointed out (rust-lang/cargo#5846) that it's
surprising that `cargo fix` (now shipping with Cargo itself!) doesn't
fix very common lint warnings, which is as good of a reminder as any
that we should finish #50723.

(Previously, we did this on the librustc and libsyntax crates in #50724. I filed cmr/this-week-in-rust#685 in hopes of recruiting new contributors to do the rest.)

r? @estebank

Align 6-week cycle check with beta promotion instead of stable release.

The regression check is to make beta promotion easier, so it makes more
sense to use the Tuesday of the release week (T-2) as the end point of the
regression prevention, instead of Thursday (T-0). But since the beta
promotion PR is sent at Tuesday evening at UTC, the protection should
include the whole Tuesday as well, meaning the 6-week cycle will start from
Wednesdays.

This will also move the start of the regression protection week one day
earlier.

travis: Attempt to debug shutdowns

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