[F&O] CER-07 exactly-one et al

F&O 5.1.1, 5.1.2, 5.1.3

The functions fn:zero-or-one, fn:one-or-more, and fn:exactly-one can trivially
be written as user functions and provide little benefit. Given widely useful
functions such as value-union that are not being provided, there seems little
justification to require these functions. Suggest they be removed.