btcec: add new type SerializedKey

This commit adds a new type called `SerializedKey`.

A serialized type is useful when using public keys as map keys. This is
because functionally identical public keys can have different internal
representations. These differences would cause the map to treat them as
different keys.

btcec/pubkey.go

@@ -51,3 +51,36 @@ type PublicKey = secp.PublicKey
 func NewPublicKey(x, y *FieldVal) *PublicKey {
 	return secp.NewPublicKey(x, y)
 }
+
+// SerializedKey is a type for representing a public key in its compressed
+// serialized form.
+//
+// NOTE: This type is useful when using public keys as keys in maps.
+type SerializedKey [PubKeyBytesLenCompressed]byte
+
+// ToPubKey returns the public key parsed from the serialized key.
+func (s SerializedKey) ToPubKey() (*PublicKey, error) {
+	return ParsePubKey(s[:])
+}
+
+// SchnorrSerialized returns the Schnorr serialized, x-only 32-byte
+// representation of the serialized key.
+func (s SerializedKey) SchnorrSerialized() []byte {
+	return s[1:]
+}
+
+// CopyBytes returns a copy of the underlying array as a byte slice.
+func (s SerializedKey) CopyBytes() []byte {
+	c := make([]byte, 33)
+	copy(c, s[:])
+
+	return c
+}
+
+// ToSerialized serializes a public key in its 33-byte compressed form.
+func ToSerialized(pubKey *PublicKey) SerializedKey {
+	var serialized SerializedKey
+	copy(serialized[:], pubKey.SerializeCompressed())
+
+	return serialized
+}