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.
I can't upload it...here's a screenshot though:
![literals](https://cloud.githubusercontent.com/assets/4156987/4837179/8b4b1598-5fd1-11e4-937d-dc011b8792df.png)

Screenshot2:
![numbers](https://cloud.githubusercontent.com/assets/4156987/4837181/910cc3dc-5fd1-11e4-9f19-83872425705e.png)