Introduces a "Syntax Index" chapter to TRPL.

Introduces a "Syntax Index" chapter to TRPL.

The intent with this chapter is to have a central place where users can
go to find out what a random bit of syntax means, be it a keyword,
symbol, or some unusual bit of composite syntax (like `for <...>`).  This
should be useful both for new users (who may not know what to call this
weird `'blah` thing), and for experienced users (who may just wish to
link someone to the appropriate section on `Trait + Trait` bounds).

Where possible, entries have been linked to an appropriate section of
the book which explains the syntax.  This was not possible in all cases.
If an entry is missing links, that's because I was unable to *find*
anything appropriate to link to.

This commit should include all stable keywords, operators and symbols,
as well as a selection of potentially confusing or unusual syntax.