[CS2] Remove unnecessary utility helper functions (#4526)

* Uncomment module test that was waiting for classes to be supported

* Rely on native indexOf

* Replace `bind` helper with native `bind` (no shortcut necessary)

* Update output

* Update modules class tests

* Remove helper for prototype extends

* Update docs to reflect removed extends operator

* Add shortcut for splice, like we have for slice

Author: GeoffreyBooth

documentation/sections/prototypal_inheritance.md

@@ -1,6 +1,6 @@
 ## Prototypal Inheritance
 
-In addition to supporting ES2015 classes, CoffeeScript provides a few shortcuts for working with prototypes. The `extends` operator can be used to create an inheritance chain between any pair of constructor functions, and `::` gives you quick access to an object’s prototype:
+In addition to supporting ES2015 classes, CoffeeScript provides a shortcut for working with prototypes. The `::` operator gives you quick access to an object’s prototype:
 
 ```
 codeFor('prototypes', '"one_two".dasherize()')