This adds an RzBuffer implementation that binds against RzIO's
rz_io_read_at/rz_io_write_at in order to access the mapped memory. This
is in contrast to the previous io RzBuffer, which has been renamed to
..._io_fd and which reads directly from a single file descriptor without
mapping.
To keep bf working, bin_bf now maps an area of zeroes because
otherwise the initial memory contents in RzIL would read as 0xff.
