age: remove Type method from Recipient and Identity interfaces

The Type() method was a mistake, as proven by the fact that I can remove
it without losing any functionality. It gives special meaning to the
"0th argument" of recipient stanzas, when actually it should be left up
to Recipient implementations to make their own stanzas recognizable to
their Identity counterparts.

More importantly, there are totally reasonable Identity (and probably
Recipient) implementations that don't know their own stanza type in
advance. For example, a proxy plugin.

Concretely, it was only used to special-case "scrypt" recipients, and to
skip invoking Unwrap. The former can be done based on the returned
recipient stanza, and the latter is best avoided entirely: the Identity
should start by looking at the stanza and returning ErrIncorrectIdentity
if it's of the wrong type.

This is a breaking API change. However, we are still in beta, and none
of the public downstreams look like they would be affected, as they only
use Recipient and Identity implementations from this package, they only
use them with the interfaces defined in this package, and they don't
directly use the Type() method.

x25519.go

@@ -34,8 +34,6 @@ type X25519Recipient struct {
 
 var _ Recipient = &X25519Recipient{}
 
-func (*X25519Recipient) Type() string { return "X25519" }
-
 // newX25519RecipientFromPoint returns a new X25519Recipient from a raw Curve25519 point.
 func newX25519RecipientFromPoint(publicKey []byte) (*X25519Recipient, error) {
 	if len(publicKey) != curve25519.PointSize {
@@ -117,8 +115,6 @@ type X25519Identity struct {
 
 var _ Identity = &X25519Identity{}
 
-func (*X25519Identity) Type() string { return "X25519" }
-
 // newX25519IdentityFromScalar returns a new X25519Identity from a raw Curve25519 scalar.
 func newX25519IdentityFromScalar(secretKey []byte) (*X25519Identity, error) {
 	if len(secretKey) != curve25519.ScalarSize {