Added Sequence.count(predicate)

Returns the number of elements in the sequence that pass the
`predicate`.

It could also have been implemented as:

  count(f) { reduce(0) {|a, b| f.call(b) ? a + 1 : a } }

But I considered the simple version more readable.

Also documented Sequence.count.

src/vm/wren_core.c

@@ -62,6 +62,16 @@ static const char* libSource =
 "    return result\n"
 "  }\n"
 "\n"
+"  count(f) {\n"
+"    var result = 0\n"
+"    for (element in this) {\n"
+"      if (f.call(element)) {\n"
+"        result = result + 1\n"
+"      }\n"
+"    }\n"
+"    return result\n"
+"  }\n"
+"\n"
 "  map(f) {\n"
 "    var result = new List\n"
 "    for (element in this) {\n"