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.

builtin/core.wren

@@ -16,6 +16,16 @@ class Sequence {
     return result
   }
 
+  count(f) {
+    var result = 0
+    for (element in this) {
+      if (f.call(element)) {
+        result = result + 1
+      }
+    }
+    return result
+  }
+
   map(f) {
     var result = new List
     for (element in this) {