internal/app/state: improve handles internals

This replaces the Store interface with something better reflecting the underlying data format for #19. An implementation of Store is provided on top of the new code to ease transition.

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

internal/app/state/store_test.go

@@ -0,0 +1,14 @@
+package state_test
+
+import (
+	"log"
+	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/internal/app/state"
+	"hakurei.app/message"
+)
+
+func TestMulti(t *testing.T) {
+	testStore(t, state.NewMulti(message.NewMsg(log.New(log.Writer(), "multi: ", 0)), check.MustAbs(t.TempDir())))
+}