Use Base._unsetindex! in pop! and popfirst! for CircularDeque

So that popped elements are not rooted by the deque and can be GCed
when they drop out of caller scope.

src/circ_deque.jl

@@ -64,6 +64,7 @@ end
 
 @inline Base.@propagate_inbounds function Base.pop!(D::CircularDeque)
     v = last(D)
+    Base._unsetindex!(D.buffer, D.last)
     D.n -= 1
     tmp = D.last - 1
     D.last = ifelse(tmp < 1, D.capacity, tmp)
@@ -91,6 +92,7 @@ Remove the element at the front.
 """
 @inline Base.@propagate_inbounds function Base.popfirst!(D::CircularDeque)
     v = first(D)
+    Base._unsetindex!(D.buffer, D.first)
     D.n -= 1
     tmp = D.first + 1
     D.first = ifelse(tmp > D.capacity, 1, tmp)