add MutableLinkedList functionality

[tmcgrath325]

Related to #443

In addition to the methods that have already been implemented for MutableLinkedList, it might also be helpful to have ways to mutate "the middle" of a list rather than only make changes at the beginning and end.

To that end, I've added popat!, insert!, and splice!.

[skygering]

I would love to bump this! I need this functionality as well!