add debug log for grasping how it works
comment in window-related debug msg
add sexplib dep to _oasis
add sexp-based print until txq/rxq (todo: user buffer)
make user buffers sexpable
add sexpable delayed ack
add a simple restore logic, but not tested (for backup)
add get_state interface (also added to V1.mli in mirage-types)
add test code
fix function sexp deserialization issue

I disregarded fun in sexpable record, serialization pretends to be fine,
but deserialization failed with raising an exception. Two functions
mattered: one is in State and the other is expire function in tcptimer.
The thing (on_close) in State is clear, but it's tricky for the tcptimer
currently used by delayed ack (ack.ml) and Tx segment (segment.ml). I
defer this issue until basic test is done, checking 'TODO-mig'.
add get/set interfaces from stackv4, which needs V1 changes
 - Currently, it strongly needs tcptimer migration, since without
   retransmission timer restoration, ret queues are accumulated...
   need debugging