Lower constant patterns with ascribed types.

Fixes #57960.

This PR fixes a bug introduced by #55937 which started checking user
type annotations for associated type patterns. Where lowering a
associated constant expression would previously return a
`PatternKind::Constant`, it now returns a `PatternKind::AscribeUserType`
with a `PatternKind::Constant` inside, this PR unwraps that to
access the constant pattern inside and behaves as before.

r? @pnkfelix

fix rustdoc JS

#58290

Add embedded book to test such that checktools works

r? @alexcrichton

FWIW this doesn't attempt to make sure we don't break check tools again -- we intentionally make the bash script not exit when x.py fails so we'd need some special case handling so for now I'm just fixing the bug itself, not its cause.

Add const generics to the AST

This is mostly split out from https://github.com/rust-lang/rust/pull/53645 in an effort to make progress merging const generics piecewise instead of in one go.

cc @yodaldevoid, @petrochenkov

r? @EddyB

Rollup of 11 pull requests

Successful merges:

 - #57504 (Re-enable history api on file:// protocol)
 - #57848 (Generate a documentation page for core::mem::transmute.)
 - #57884 (Update minifier version)
 - #57954 (rustdoc: remove blank unstable spans)
 - #58028 (Fix image link in the settings menu)
 - #58033 (rustdoc: wrap stability tags in colored spans)
 - #58086 ([rustdoc] Improve file list display)
 - #58143 (Sort elements in the sidebar)
 - #58146 (Prevent automatic collapse of methods impl blocks)
 - #58150 (Don't apply impl block collapse rules to trait impls)
 - #58185 (Remove images' url to make it work even without internet connection)

Failed merges:

r? @ghost

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Co-Authored-By: NGabriel Smith <yodaldevoid@users.noreply.github.com>

Remove images' url to make it work even without internet connection

Needed for local std docs mainly.

cc @SimonSapin

r? @QuietMisdreavus

Rollup merge of #58150 - GuillaumeGomez:dont-apply-impl-collapse-rules-to-trait-impls, r=Manishearth

Don't apply impl block collapse rules to trait impls

Fixes #58147.

r? @QuietMisdreavus

Prevent automatic collapse of methods impl blocks

Fixes #57582.

r? @QuietMisdreavus

Sort elements in the sidebar

Fixes #57448.

Not sorting fields or variants though.

r? @QuietMisdreavus

[rustdoc] Improve file list display

Improve a bit the arrows to show a folder's file and the arrow to show/hide the left panel.

r? @QuietMisdreavus

rustdoc: wrap stability tags in colored spans

A cosmetic change to make the stability tags stand out a bit against the docs. Opening for discussion.

Before:

![screen shot 2019-01-31 at 3 29 36 pm](https://user-images.githubusercontent.com/1372438/52083406-54730d80-256d-11e9-8e61-b8caff569434.png)
![screen shot 2019-01-31 at 3 31 32 pm](https://user-images.githubusercontent.com/1372438/52083408-54730d80-256d-11e9-97b7-43e808448f65.png)

After:
![screen shot 2019-01-31 at 3 29 18 pm](https://user-images.githubusercontent.com/1372438/52083405-54730d80-256d-11e9-9983-19d9519b2ed8.png)
![screen shot 2019-01-31 at 3 29 46 pm](https://user-images.githubusercontent.com/1372438/52083407-54730d80-256d-11e9-8c32-11a1ad7d3f34.png)

r? @QuietMisdreavus