feat(stdlib): Add mutable doubly-linked list

[alex-snezhko]

Closes #1815

Wherever I thought it made sense, I made the list functions mutate the given list rather than producing a new list e.g. append

[spotandjake]

This implementation looks really solid. a few little documentaiton issues i noticed.

[spotandjake]

Looks Good To Me.

[marcusroberts]

Amazing! People can start to teach Computer Science using Grain now.

This would make a great example of some idiomatic Grain!

[peblair]

The implementation looks good here, but I have some nits with the high-level design (maybe I missed a conversation):

1. I think this module should be called Deque, not Mutable. For one, it's not just a mutable singly-linked list, which is what I would expect List.Mutable to be. Second, most languages (e.g. Java and Python) call these structures Deques.
2. With that change, I think it makes sense to pull this module out of list.gr and put it in its own file.

[alex-snezhko]

@peblair True, but I think an argument in favor of keeping it as a member of the List module is that the interface for this type extends beyond the typical scope of a deque as an abstract data type, notably adding/removing/querying from only the beginning or end of the structure. There are indeed some languages like Python whose collections.deque has more capabilities than just these, but in my view that betrays the purpose of a deque.

I also think in this same way you can draw a parallel between Grain's List and Stack, where the only ways you can "modify" a List (adding to and removing from the front) capture the behavior of a stack, but the other functions made available to the List module like map, etc make it philosophically more general-purpose even though on its face a List is just a stack.

What do you think?