chore(all): return concrete types, accept interfaces

- Remove exported interfaces unused locally
- Define interfaces to accept arguments
- Return concrete types, not interfaces

internal/loopstate/lock.go

@@ -1,9 +1,4 @@
 package loopstate
 
-type Locker interface {
-	Lock()
-	Unlock()
-}
-
 func (s *State) Lock()   { s.loopMu.Lock() }
 func (s *State) Unlock() { s.loopMu.Unlock() }