`const` before destructuring is inconsistent

```
julia> const (a,) = (1,2,3)
(1, 2, 3)

julia> a
1

julia> isconst(@__MODULE__, :a)
true
```
vs
```

julia> const (b...,) = (1,2,3)
(1, 2, 3)

julia> b
(1, 2, 3)

julia> isconst(@__MODULE__, :b)
false
```
I'm not really a fan of these complex destructurings in const at all, but they should at least be consistent.