feat: allow empty argument list methods in calls, definitions, and docs

- Compile empty `()` as a valid method signature distinct from no parentheses
- Update `call()`, `clear()`, `run()`, `try()`, and `yield()` to use empty argument lists
- Revise documentation across multiple files to reflect new signature semantics

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