:vis matcher for macro_rules

Resurrection of @DanielKeep's implementation posted with [RFC 1575](rust-lang/rfcs#1575).

@jseyfried was of the opinion that this doesn't need an RFC.

Needed before merge:

- [x] sign-off from @DanielKeep since I stole his code
- [x] feature gate
- [x] docs

…laumeGomez

rustdoc: add a list of headings to the sidebar

It's another misdreavus rustdoc PR, which means it's time for Bikeshed City once again! `:3`

In an effort to aid navigation in long documentation pages, this PR adds a listing of headings to the sidebars of pages where such headings exist. For example, for structs, links to their fields, inherent methods, and trait implementations are available where applicable.

Examples:

* Modules/Crate roots
  ![image](https://cloud.githubusercontent.com/assets/5217170/25019930/1000fa3a-2052-11e7-98ff-ddf5af18b3e6.png)
* Enums
  ![image](https://cloud.githubusercontent.com/assets/5217170/25019954/33497f9e-2052-11e7-88cf-df46f1b3b8a3.png)
* Primitives
  ![image](https://cloud.githubusercontent.com/assets/5217170/25019978/4820bbc6-2052-11e7-8b5a-96a864eb2a5b.png)
* Traits
  ![image](https://cloud.githubusercontent.com/assets/5217170/25020093/bd1bc9f2-2052-11e7-9cd1-00a0ad8007bc.png)
* Structs
  ![image](https://cloud.githubusercontent.com/assets/5217170/25020145/d75206b0-2052-11e7-88cc-9e9525084775.png)

Open questions:

* Right now, these kinds of pages (and also unions) are the only pages that will receive the name header - pages for functions, constants, macros, etc, won't have the corresponding name in their sidebar. Should I print the name regardless and only add table-of-contents links for pages that have them? This would make them match, for example, a struct with no public fields, no methods, and no trait implementations. The latter would still have a "Struct MyStruct" line above the module contents, with no header links to speak of, whereas a function wouldn't even have "Function my\_function".
* This is only a header listing, but there has been requests to include a more-complete listing of fields/methods/traits/etc, for example in rust-lang#41123.

…steveklabnik

Hoedown big comeback!

```bash
> cargo +local test
   Compiling libc v0.2.20
   Compiling sysinfo v0.3.4 (file:///Users/imperio/rust/sysinfo)
    Finished dev [unoptimized + debuginfo] target(s) in 3.2 secs
     Running target/debug/deps/disk_list-dbd70897f1f7e080

running 1 test
test test_disks ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

     Running target/debug/deps/sysinfo-8ad11103abdf5941

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

   Doc-tests sysinfo
WARNING: src/sysinfo.rs -  (line 45) test will be run in the next rustdoc version. If it's not supposed to, please update your documentation and make it compliant to common mark specifications.
WARNING: src/sysinfo.rs -  (line 48) test will be run in the next rustdoc version. If it's not supposed to, please update your documentation and make it compliant to common mark specifications.

running 1 test
test src/sysinfo.rs -  (line 14) ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
```

r? @rust-lang/docs