[WIP] Tried to add temporary immutable struct

[AliMalik9599]

Hi, I'm relatively new to Julia and was interested in this problem. I attempted to add a temporary way to make queue immutable, but definitely a work in progress and needs changes. Any tips are appreciated, thanks.

[oxinabox]

making Queue a immutable struct should be just a matter of removing the mutable, and making sure all operations just mutate the backing Deque instead. That would be a good idea.