Adds LinkedList declaration.
Adds LinkedList tests.
Implements all LinkedList functions except for Insert and Remove.
Adds tests for all LinkedList functions but Iterate.
Adds correct length-accounting code to LinkedLists's Append and Prepend.

Maybe I really should revert to making length getters O(n)...
Implements LinkedList Insert and Remove.
Fixes bug preventing proper building.
Adds test for LinkedList iteration.