Allow empty argument list methods.

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

test/break/closure_in_for.wren

@@ -5,5 +5,5 @@ for (i in [1, 2, 3]) {
   break
 }
 
-f.call
+f.call()
 // expect: 5