Allow empty argument list methods.

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

test/for/close_over_loop_variable.wren

@@ -4,7 +4,7 @@ for (i in [1, 2, 3]) {
   list.add(new Fn { IO.print(i) })
 }
 
-for (f in list) f.call
+for (f in list) f.call()
 // expect: 1
 // expect: 2
 // expect: 3

test/for/closure_in_body.wren

@@ -5,7 +5,7 @@ for (i in [1, 2, 3]) {
   list.add(new Fn { IO.print(j) })
 }
 
-for (f in list) f.call
+for (f in list) f.call()
 // expect: 2
 // expect: 3
 // expect: 4

test/for/only_evaluate_sequence_once.wren

@@ -3,7 +3,7 @@ var f = new Fn {
   return [1, 2, 3]
 }
 
-for (i in f.call) IO.print(i)
+for (i in f.call()) IO.print(i)
 // expect: evaluate sequence
 // expect: 1
 // expect: 2

test/for/return_closure.wren

@@ -4,6 +4,6 @@ var f = new Fn {
   }
 }
 
-var g = f.call
-g.call
+var g = f.call()
+g.call()
 // expect: 1

test/for/return_inside.wren

@@ -4,5 +4,5 @@ var f = new Fn {
   }
 }
 
-IO.print(f.call)
+IO.print(f.call())
 // expect: 1