Allow empty argument list methods.

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

test/return/after_else.wren

@@ -1,3 +1,3 @@
 IO.print(new Fn {
   if (false) "no" else return "ok"
-}.call) // expect: ok
+}.call()) // expect: ok

test/return/after_if.wren

@@ -1,3 +1,3 @@
 IO.print(new Fn {
   if (true) return "ok"
-}.call) // expect: ok
+}.call()) // expect: ok

test/return/after_while.wren

@@ -1,3 +1,3 @@
 IO.print(new Fn {
   while (true) return "ok"
-}.call) // expect: ok
+}.call()) // expect: ok

test/return/in_function.wren

@@ -3,4 +3,4 @@ var f = new Fn {
   IO.print("bad")
 }
 
-IO.print(f.call) // expect: ok
+IO.print(f.call()) // expect: ok

test/return/return_null_if_newline.wren

@@ -3,4 +3,4 @@ var f = new Fn {
   IO.print("bad")
 }
 
-IO.print(f.call) // expect: null
+IO.print(f.call()) // expect: null