Merge pull request #220 from bjorn/count_predicate

Added Sequence.count(predicate)

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(_)'.