Make casewriters keep track of the number of `union value's in each

Make casewriters keep track of the number of `union value's in each
case.  This is useful for two reasons: casewriter_write can then check
that the case being written is large enough, and later recipients of
the casewriter can determine the size of the case.

* get.c (case_map_get_value_cnt): New function.

* casewriter-translator.c (casewriter_create_translator): Add
value_cnt parameter.

* casewriter.c (struct casewriter): Add value_cnt member.
(casewriter_write): Check that the case passed in is big enough.
(casewriter_get_value_cnt): New function.
(casewriter_create): Add value_cnt parameter.