Allow empty argument list methods.

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

test/assignment/local.wren

@@ -7,4 +7,4 @@ new Fn {
 
   IO.print(a = "arg") // expect: arg
   IO.print(a) // expect: arg
-}.call
+}.call()