all: implement AppendText alongside MarshalText (#9207)

This eventually allows encoding packages that may respect
the proposed encoding.TextAppender interface.
The performance gains from this is between 10-30%.

Updates tailscale/corp#14379

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

types/key/disco.go

@@ -127,9 +127,14 @@ func (k DiscoPublic) String() string {
 	return string(bs)
 }
 
+// AppendText implements encoding.TextAppender.
+func (k DiscoPublic) AppendText(b []byte) ([]byte, error) {
+	return appendHexKey(b, discoPublicHexPrefix, k.k[:]), nil
+}
+
 // MarshalText implements encoding.TextMarshaler.
 func (k DiscoPublic) MarshalText() ([]byte, error) {
-	return toHex(k.k[:], discoPublicHexPrefix), nil
+	return k.AppendText(nil)
 }
 
 // MarshalText implements encoding.TextUnmarshaler.