com.docker.slirp: replace `Forward` with `Instance`
com.docker.slirp: replace `Forward` with `Instance`

This verifies that we're using the types from the functor argument
and not sneaking a peak around the side at our `module Forward`.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit,samoht/vpnkit