Allow empty argument list methods.

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

test/nonlocal/assignment.wren

@@ -14,5 +14,5 @@ IO.print(Nonlocal) // expect: method
 
 new Fn {
   Nonlocal = "fn"
-}.call
+}.call()
 IO.print(Nonlocal) // expect: fn

test/nonlocal/use_in_function.wren

@@ -2,4 +2,4 @@ var Global = "global"
 
 new Fn {
   IO.print(Global) // expect: global
-}.call
+}.call()

test/nonlocal/use_in_function_before_definition.wren

@@ -4,4 +4,4 @@ var f = new Fn {
 
 var Global = "global"
 
-f.call // expect: global
+f.call() // expect: global