Move MasterKey to separate package to avoid import cycles

keys/keys.go

@@ -0,0 +1,11 @@
+package keys
+
+// MasterKey provides a way of securing the key used to encrypt the Tree by encrypting and decrypting said key.
+type MasterKey interface {
+	Encrypt(dataKey []byte) error
+	EncryptIfNeeded(dataKey []byte) error
+	Decrypt() ([]byte, error)
+	NeedsRotation() bool
+	ToString() string
+	ToMap() map[string]interface{}
+}