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/resume_caller.wren

@@ -4,12 +4,12 @@ var b = new Fiber {
 
 var a = new Fiber {
   IO.print("begin fiber a")
-  b.call
+  b.call()
   IO.print("end fiber a")
 }
 
 IO.print("begin main")
-a.call
+a.call()
 IO.print("end main")
 
 // expect: begin main