more information in transition label (trans_type)

[amutake]

and rename trans_type -> label

Inductive label :=
| Receive dest source content (* `dest` receives a message `content` from `source` *)
| Send source dest content    (* `source` sends a message `content` to `dest` *)
| New parent child            (* `parent` creates an actor named `child` *)
| Self myname                 (* `myname` gets own name *)

This is required for fairness (enabled definition). #8

[amutake]

#10