Provide a backend implementation for heap storage

[svenstaro]

The current implementations for backend storage accept only stack-allocated arrays, either as a reference or moved. I think it would be neat as some embedded hardware supports this though with a warning that this might not be supported everywhere.

My problem is that my buffers are 320x240x16b and so I need around 160kB per buffer (and I have two of them) which is quite the squeeze in stack memory.

[jounathaen]

I think #23 fixes this by allowing slices as backend.