Allow empty argument list methods.

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

doc/site/index.markdown

@@ -18,7 +18,7 @@ a familiar, modern [syntax][].
       ["small", "clean", "fast"].map {|word| Fiber.yield(word) }
     }
 
-    while (!adjectives.isDone) IO.print(adjectives.call)
+    while (!adjectives.isDone) IO.print(adjectives.call())
 
  *  **Wren is small.** The codebase is about [5,000 lines][src]. You can
     skim the whole thing in an afternoon. It's *small*, but not *dense*. It