Allow empty argument list methods.

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

test/while/closure_in_body.wren

@@ -7,7 +7,7 @@ while (i < 4) {
   i = i + 1
 }
 
-for (f in list) f.call
+for (f in list) f.call()
 // expect: 2
 // expect: 3
 // expect: 4

test/while/return_closure.wren

@@ -5,6 +5,6 @@ var f = new Fn {
   }
 }
 
-var g = f.call
-g.call
+var g = f.call()
+g.call()
 // expect: i

test/while/return_inside.wren

@@ -5,5 +5,5 @@ var f = new Fn {
   }
 }
 
-IO.print(f.call)
+IO.print(f.call())
 // expect: i