Change linked-lists tests and example to use mutation.

[theprash]

For #240.

I also fixed an error in pop and shift where next/prev elements weren't being set to None and the list was keeping references to items that had been removed. The tests do not check for this.

[ErikSchierboom]

This looks great! Thanks.

I have one small request: could you add a HINTS.md file to the exercise's directory? In this file we should link to a page that helps users understand how to use classes in F#. I think this page would be a great resource to include. You can see an example of how such an HINTS.md file looks like here.

[theprash]

Why classes? The example implementation still just uses records with mutable fields. I was thinking about explaining the mutable keyword and linking to a resource.

[theprash]

I've added hints mentioning both records and classes.

[ErikSchierboom]

Oh sorry, I should have of course mentioned records too. I think the hints file is perfect. Merged!

Thanks a lot for all the work! 🎉

[robkeim]

Nice job @theprash these changes look good!