Allow empty argument list methods.

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

test/for/closure_in_body.wren

@@ -5,7 +5,7 @@ for (i in [1, 2, 3]) {
   list.add(new Fn { IO.print(j) })
 }
 
-for (f in list) f.call
+for (f in list) f.call()
 // expect: 2
 // expect: 3
 // expect: 4