"is" operator.

test/is.wren

@@ -0,0 +1,16 @@
+io.write(true is Bool) // expect: true
+io.write((fn 1) is Function) // expect: true
+io.write(123 is Num) // expect: true
+io.write(null is Null) // expect: true
+io.write("s" is String) // expect: true
+
+io.write(Num is Bool) // expect: false
+io.write(null is Class) // expect: false
+io.write(true is Function) // expect: false
+io.write((fn 1) is Num) // expect: false
+io.write("s" is Null) // expect: false
+io.write(123 is String) // expect: false
+
+// TODO(bob): Non-class on RHS.
+// TODO(bob): Precedence and associativity.
+// TODO(bob): Metaclasses ("Num is Class").