hst/instance: define instance state

This is now part of the hst API. This change also improves identifier generation and serialisation.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/process.go

@@ -289,7 +289,7 @@ func (k *outcome) main(msg message.Msg) {
 
 	// shim accepted setup payload, create process state
 	if ok, err := ms.store.Do(k.state.identity.unwrap(), func(c state.Cursor) {
-		if err := c.Save(&state.State{
+		if err := c.Save(&hst.State{
 			ID:     k.state.id.unwrap(),
 			PID:    ms.cmd.Process.Pid,
 			Config: k.config,