The multikey module provides a BTreeMap implementation that can use one
of 2 different kinds of keys to look up a value. This is needed by the
virtio-fs server since it needs to be able to look up keys either by
u64 or by a (ino_t, dev_t) pair.
This code has been ported over from crosvm commit
961461350c0b6824e5f20655031bf6c6bf6b7c30.
Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>