Move the memory allocator outside the Lwt module

There's nothing Lwt-specific here.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add an interface for the Memory module

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Memory: hide more implementation details

Signed-off-by: David Scott <dave.scott@eu.citrix.com>