Adding to list API

Expanding list API to support setting and getting values at specific indices.
In total, the list API will be nearly identical to the seq API.

Searching for arbitrary indices in a linked-list is inefficient, hence the need
for seq.  However, if I can improve this search efficiency (for instance, by
keeping a set of pointers that point to every n'th node, thereby bounding
the inefficiency), I will remove seq altogether.  I'd like to keep the number
of datastructures small.