Fix broadcast()

[nalimilan]

We really need to fix this:

julia> isnull.(NullableArray(1:3))
ERROR: MethodError: no method matching broadcast_shape(::NullableArrays.NullableArray{Int64,1})
Closest candidates are:
  broadcast_shape(::Tuple) at broadcast.jl:53
  broadcast_shape(::Tuple, ::Tuple, ::Tuple...) at broadcast.jl:54
 in broadcast(::Function, ::NullableArrays.NullableArray{Int64,1}) at ./<missing>:0

[davidagold]

We do. I'm pretty tired of trying to track changes to broadcast in Base. I vote that we just stop hosting our own broadcast implementation entirely, and rely on the AbstractArray interface. AFAICT, the only things we'd lose are

1. some performance since we're no longer operating directly on the :values and :isnull fields of the argument NullableArrays
2. the lift keyword argument

I'm fine with both of those for now.

[nalimilan]

I don't have the experience of working with that code myself, so I can't really tell what's best. But indeed if we can't fix it soon, better rely on the Base implementation.

Also, regarding 1, I would have thought the compiler would be able to optimize getindex away. But I haven't checked (and I think you did). Regarding 2, your lift function could be a good replacement (and a more general one).

[davidagold]

I haven't checked. I need to learn how to read lowered code. And yes, I intend the higher-order lift function to be a solution for this case, since writing broadcast(f, X, lift=true) is not any more convenient than writing broadcast(lift(f), X).

[nalimilan]

Turns out these functions were simply renamed (not sure that was a good idea...). See #164.

So we can wait until we're more confident about removing the special methods for broadcast.