KV_RO: read takes offset, length and returns a buffer list

This makes KV_RO.read the same as FS.read
KV_RO type error is now a polymorphic variant, as is the convention
Include KV_RO as a submodule within the module type FS

Unfortunately 'include KV_RO' fails because of duplicate definitions
of the "type 'a io"

Did I miss some module include magic?