make "happly" a parsing notation for "ap10"

@marcbezem pointed out that the book uses `happly` for our `ap10` but nowhere is it mentioned that they are the same.  It would be nice to mention this in `HoTTBook.v` as well, but the definition of `happly` in the book is not a theorem or definition, it's _equation_ (2.9.2), and thus doesn't appear therein.  What can be done?