Allow empty argument list methods.

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

test/nonlocal/assignment.wren

@@ -14,5 +14,5 @@ IO.print(Nonlocal) // expect: method
 
 new Fn {
   Nonlocal = "fn"
-}.call
+}.call()
 IO.print(Nonlocal) // expect: fn