Change syntax for blocks -> fns.

test/fn_equality.wren

@@ -0,0 +1,16 @@
+// Not structurally equal.
+io.write((fn 123) == (fn 123))  // expect: false
+io.write((fn 123) != (fn 123))  // expect: true
+
+// Not equal to other types.
+io.write((fn 123) == 1)         // expect: false
+io.write((fn 123) == false)     // expect: false
+io.write((fn 123) == "fn 123")  // expect: false
+io.write((fn 123) != 1)         // expect: true
+io.write((fn 123) != false)     // expect: true
+io.write((fn 123) != "fn 123")  // expect: true
+
+// Equal by identity.
+var f = fn 123
+io.write(f == f) // expect: true
+io.write(f != f) // expect: false