chore(all): return concrete types, accept interfaces

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

internal/tun/create.go

@@ -8,10 +8,6 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-type Creator interface {
-	Create(path string) error
-}
-
 // Create creates a TUN device at the path specified.
 func (t *Tun) Create(path string) error {
 	parentDir := filepath.Dir(path)