Allow empty argument list methods.

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

test/fiber/run_with_value_indirect_reenter.wren

@@ -13,7 +13,7 @@ b = new Fiber {
   IO.print(3)
 }
 
-b.call
+b.call()
 // expect: 1
 // expect: 2
 // expect: 3