Allow empty argument list methods.

- Compile them as calls and definitions.
- Use them for call(), clear(), run(), try(), and yield().
- Update the docs.

test/closure/reuse_closure_slot.wren

@@ -10,7 +10,7 @@
     // Since a is out of scope, the local slot will be reused by b. Make sure
     // that f still closes over a.
     var b = "b"
-    f.call // expect: a
+    f.call() // expect: a
   }
 }