Wherever possible just forward readblock!

[rafaqz]

It seems in a lot of cases we call getindex on the parent objects rather than just jumping straight to readblock. If for example we know the type doesn't change we don't always need to allocate another output buffer.

Calling readblock! rather than getindex also has a much lower compilation overhead.

[rafaqz]

I think we need to define generic readblock on AbstractArray` for this to work generally, it broke a lot of things when I tried it locally.