Add an .lwt subpackage, port the nascent framebuffer example to lwt.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Lwt_unix.write can return short, retry.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
framebuffer: perform updates in the background

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