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/shadow_closure_with_local.wren

@@ -7,5 +7,5 @@
       IO.print(foo) // expect: shadow
     }
     IO.print(foo) // expect: closure
-  }.call
+  }.call()
 }