rustdoc currently determines whether to produce a table of
contents (along with numbered sections) from the input type: yes for
markdown input, no for Rust input. This commit adds a flag to disable
the table of contents for markdown input, which is useful for embedding
the output in a larger context.

Use insert/remove instead.

This adds an `--extern` flag to `rustdoc` much like the compiler's to
specify the path where a given crate can be found.

Updated documentation to reflect md->html.
Modularized external file loading.

This commit removes superfluous to_string calls from various places

This grows a new option inside of rustdoc to add the ability to submit examples
to an external website. If the `--markdown-playground-url` command line option
or crate doc attribute `html_playground_url` is present, then examples will have
a button on hover to submit the code to the playground specified.

This commit enables submission of example code to play.rust-lang.org. The code
submitted is that which is tested by rustdoc, not necessarily the exact code
shown in the example.

Closes #14654

These were only used for the markdown tests, and there's no reason they should
be distinct from the other tests.

[breaking-change]

[breaking-change]

Some straggling instances of `~[]` across a few different libs. Also,
remove some public ones from workcache.

port all code over to use it.

There's a little more allocation here and there now since
from_utf8_owned can't be used with Vec.

`~[T]` in test, libgetopts, compiletest, librustdoc, and libnum.

It's now in the prelude.

Closes #12771

- fixup and refactor highlighting code
- have a proper print stylesheet

The manual, tutorial and guides need the feature gates quite often,
unfortunately, so this is the low-cost path to migrating to use
rustdoc. This is only activated for pure-Markdown files.

Preferably this would be avoided: #12773

markdown files.

This means that

    # Foo
    ## Bar
    # Baz
    ### Qux
    ## Quz

Gets a TOC like

    1 Foo
       1.1 Bar
    2 Baz
       2.0.1 Qux
       2.1 Quz

This functionality is only used when rendering a single markdown file,
never on an individual module, although it could very feasibly be
extended to allow modules to opt-in to a table of contents (std::fmt
comes to mind).

This theoretically gives rustdoc the ability to render our guides,
tutorial and manual (not in practice, since the files themselves need to
be adjusted slightly to use Sundown-compatible functionality).

Fixes #11392.