add to_s, [], []=, insert, delete, concat, and dup to linked list
refactor ll to use entry#link
add reverse method to linked list
add reverse_bang, dqueue, and entry#to_s to linked list
rewrite as singly linked list, write doubly linked list to inherit from singly linked list
separated classes into separate files, created lib structure
add hash to all heap methods, add delete method, fix max/min, refactor add children and parent methods