Allow empty argument list methods.

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

test/this/closure.wren

@@ -4,4 +4,4 @@ class Foo {
 }
 
 var closure = (new Foo).getClosure
-IO.print(closure.call) // expect: Foo
+IO.print(closure.call()) // expect: Foo

test/this/nested_class.wren

@@ -13,7 +13,7 @@ class Outer {
       }
 
       (new Inner).method
-    }.call
+    }.call()
   }
 
   toString { "Outer" }

test/this/nested_closure.wren

@@ -4,4 +4,4 @@ class Foo {
 }
 
 var closure = (new Foo).getClosure
-IO.print(closure.call.call.call) // expect: Foo
+IO.print(closure.call().call().call()) // expect: Foo