2.6: Deprecate xbind-related things in Mapper.

This includes a bunch of field binding and form snippet
helpers. New versions are added for doing similar things with
CssSels instead.

They don't map 1-to-1 because CSS selector transforms don't
support namespaces, amongst other things. Transforms are
combined using `andThen` in case they match the same element
or match a parent element and something else needs to affect
the child. If folks think `&` would be a better way to go, I'm totally
willing to go for it—I vacillated a bit between the two.