ARELLE-267, periodEnd should return set, not list
ARELLE-268, custom function signatures indexed both by qname alone
and by qname, arity
ARELLE-268, output facts of type derived from xml:decimal
weren't truncating at precision/decimal (as float was before)
ARELLE-268 custom functions indexed by qname and arity for finding
right implementation.  Test case 60200v13 added.