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.

test/core/list/count_predicate.wren

@@ -0,0 +1,6 @@
+var a = [1, 2, 3]
+
+IO.print(a.count {|x| x > 3 })      // expect: 0
+IO.print(a.count {|x| x > 1 })      // expect: 2
+
+IO.print([].count {|x| true })      // expect: 0

test/core/list/count_predicate_non_bool_returning_fn.wren

@@ -0,0 +1,3 @@
+var a = [1, 2, 3]
+
+IO.print(a.count {|x| "truthy" })   // expect: 3

test/core/list/count_predicate_non_function_arg.wren

@@ -0,0 +1,3 @@
+var a = [1, 2, 3]
+
+a.count("string") // expect runtime error: String does not implement 'call(_)'.