Add skeleton for memory backend

Memory backend will be used as the primary test for further backend
implementation within Buxton, and will cause various API changes to
ensure the system is well integrated.

The following commits will have to restructure the existing API to
make backends unaware of explicit datatypes, migrating to set/get
value methods, typed via struct/union keys.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>