midx: shun buffers

Rework PackMidx to avoid buffers which are reasonably heavyweight, and
will be even larger (as memoryviews) in python 3, requiring ~200
bytes.  Instead, just use direct offsets into the underlying mmap --
slicing an mmap currently just produces bytes.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>