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 assumed the keyword table is odd because this is what it looks like for me:

![keywords](https://cloud.githubusercontent.com/assets/4156987/4837400/570983ec-5fd5-11e4-899c-80f321f5e8c6.png)