I'd like to suggest some improvements from mmap

1) mmap assign to slice only accepts a string.  This is unfortunate,
because AFAIK a string can only be created by copying data, and this is
wasteful for large data transfers.  mmap should accept any object
supporting buffer protocol as well as string.

2) buffer (mmap_obj) gives a read_only buffer.  There should be a way to
make this read_write.  I suggest 'as_buffer' member.

3) mmap_obj does not support weak ref.  This is needed for proper
lifetime management using boost::python