Move various implementation classes into the 'Imp' namespace

There are various classes in dune-istl that are effectively implementation
classes internal to dune-istl. A few of them are base_array_unmanaged,
base_array_window, compressed_base_array_unmanaged,block_vector_unmanaged,
BlockVectorWindow, compressed_block_vector_unmanaged, CompressedBlockVectorWindow

See #22 for a brief
discussion.

This patch moves the classes into the namespace 'Imp'.  That way it is
clear that they are internal.

All code outside of dune-istl using those classes will break, but does
such code really exist?