switch around the WithCheck function

It should be better with an interface. That way it can be written as

ensure.WithCheck(resource)

main.go

@@ -17,22 +17,27 @@ type (
 		Ensure() error
 	}
 
+	// CheckEnsurer is an etension of ensurer, which also provides an Is function
+	// to check the state of the resource.
+	CheckEnsurer interface {
+		Is() bool
+		Ensure() error
+	}
+
 	checkEnsurer struct {
-		Is    Enforced
+		State CheckEnsurer
-		State Ensurer
 	}
 )
 
 // WithCheck creates an Ensurer instance that runs is before Ensure.
-func WithCheck(is Enforced, en Ensurer) Ensurer {
+func WithCheck(en CheckEnsurer) Ensurer {
 	return &checkEnsurer{
-		Is:    is,
 		State: en,
 	}
 }
 
 func (c *checkEnsurer) Ensure() error {
-	if !c.Is() {
+	if !c.State.Is() {
 		return c.State.Ensure()
 	}
 	return nil