Collect literal examples together in the reference

Fixes #18415

This links [`std::str`](http://doc.rust-lang.org/std/str/index.html) documentation to [literals](http://doc.rust-lang.org/reference.html#literals) in the reference guide and collects examples of literals into one group at the beginning of the section. The new tables are not exhaustive (some escapes were skipped) and so I try to link back to the respective sections where more detail is located.

I don't think the tables actually look that nice if that's important and I'm not sure how it could be improved. I think it does do a good job of collecting available options together. I think listing the escapes together is particularly helpful because they vary with type and are embedded in paragraphs.
Yeah. The Github [markdown example](https://guides.github.com/features/mastering-markdown/) shows the first pipe aligned. The rest are automatic. Like so:

```
First Header | Second Header
------------ | -------------
Content from cell 1 | Content from cell 2
Content in the first column | Content in the second column
```
First Header | Second Header
------------ | -------------
Content from cell 1 | Content from cell 2
Content in the first column | Content in the second column