Sum of a Queue is type unstable

[HenriDeh]

Example :

julia> using DataStructures

julia> q=Queue{Float64}()
Queue{Float64}(Deque [Float64[]])

julia> enqueue!(q, 0)
Queue{Float64}(Deque [[0.0]])

julia> enqueue!(q, 1)
Queue{Float64}(Deque [[0.0, 1.0]])

julia> enqueue!(q, 3)
Queue{Float64}(Deque [[0.0, 1.0, 3.0]])

julia> @code_warntype sum(q)
Variables
  #self#::Core.Compiler.Const(sum, false)
  a::Queue{Float64}

Body::Any
1 ─ %1 = Base.sum(Base.identity, a)::Any
└──      return %1
```

Same for Deque:

```
@code_warntype sum(q.store)
Variables
  #self#::Core.Compiler.Const(sum, false)
  a::Deque{Float64}

Body::Any
1 ─ %1 = Base.sum(Base.identity, a)::Any
└──      return %1
```

[Ellipse0934]

Close this ?