Boundscheck blocks for SubString should perhaps be propagated

```
julia> f(s) = @inbounds SubString(s, 1, 5)
fff (generic function with 1 method)

julia> f("dsf")
ERROR: BoundsError: attempt to access "dsf"
  at index [1:5]
```

Perhaps the boundscheck from the outer constructor should be propagated to the inner constructor.
