buffer,crypto,osd,tools: extract digest methods out of bufferlist

buffer,crypto,osd,tools: extract digest methods out of bufferlist

they were consumers of ceph::list::buffers(), and there is no need to
make them member functions of ceph::list, so move them into ceph::crypto
namespace. and update the unit test and ceph_dedup_tool.cc accordingly.

Signed-off-by: Kefu Chai <kchai@redhat.com>