Implement fn:exactly-one.

There are two phases to this. One behaves like

 if (fn:exists(x)) then x else error 

. This is the version that we need first.

Later, once DFDL-1118 is implemented this needs to be revisited. However, that's a separate issue. Not even creating a ticket for it. This task should be part of the implementation work for DFDL-1118.