Change #contents to #in

After actual use I realize `#contents` is used a LOT.

Typing so much is impractical and verbose, not to mention the
oversized line-lengths.

`#in` is the new-and-improved version. Much shorter and manages to
keep the meaning clear. (There weren't really that many options
to replace `#contents`. I checked a thesaurus.)
Use htmlspecialchars in _render_attribute_values()
Add error messages if certain items are not renderable arrays
Handle multiple callbacks