Add Dict methods to LRUCache

[imreddyTeja]

Purpose

Adds expected Dict methods to LRUCache Closes #79 -- this will automatically close issue 79 on PR merge

To-do

Content

Add haskey, copy, deepcopy, empty, empty!, get, pop!, getindex, setindex!, and delete! methods to LRUCache

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• [x] I have read and checked the items on the review checklist.