The array to scan.
The evaluation predicate, which should return true if the element is a valid match and false if not. This predicate may be any callable type.
The number of elements where pred returns true.
test!("==")(countIf("gbbbi", ( char c ) { return c == 'b'; }), 3);
Performs a linear scan of haystack from [0 .. haystack.length$(RP), returning a count of the number of elements where pred returns true.