multigrab/fastsplit: Plumb ShowUnocculted and &PieceTrait

multigrab/fastsplit: Plumb ShowUnocculted and &PieceTrait

The implementation doesn't really want PieceRenderingInstructions,
just permission to get at trait objects.

The currency fastsplit implementation callback is going to want to
look at its Banknote so that it can make a nice log message.  I think
fastsplit piece impls are entitled access to their concret piece type.
So downcast it, and pass it down the layers.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>