Collect literal examples together in the reference

Fixes #18415

This links [`std::str`](http://doc.rust-lang.org/std/str/index.html) documentation to the literals part of 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.