Fix runtime dispatch in Deque

[Mellich-2]

Hi,

In the current version 0.18.20 of DataStructures, using dequeue! leads to runtime dispatches affecting performance.
The error can be fixed by declaring a used alias as const.

Small example reproducing the error:

using JET
using Pkg
Pkg.activate(".")
using DataStructures
q = Queue{Int}()
enqueue!(q, 1)
@report_opt dequeue!(q)

results in

´´´
═════ 1 possible error found ═════
┌ dequeue!(s::Queue{Int64}) @ DataStructures /home/marius/repos/DataStructures.jl/src/queue.jl:37
│┌ popfirst!(q::Deque{Int64}) @ DataStructures /home/marius/repos/DataStructures.jl/src/deque.jl:305
││ runtime dispatch detected: %33::Any(%34::Vector{Int64}, %35::Int64)::Any
│└────────────────────
´´´

[oxinabox]

thanks!