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/util.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"slices"
 
 	"go4.org/mem"
 )
@@ -49,11 +50,19 @@ func clamp25519Private(b []byte) {
 	b[31] = (b[31] & 127) | 64
 }
 
-func toHex(k []byte, prefix string) []byte {
-	ret := make([]byte, len(prefix)+len(k)*2)
-	copy(ret, prefix)
-	hex.Encode(ret[len(prefix):], k)
-	return ret
+func appendHexKey(dst []byte, prefix string, key []byte) []byte {
+	dst = slices.Grow(dst, len(prefix)+hex.EncodedLen(len(key)))
+	dst = append(dst, prefix...)
+	dst = hexAppendEncode(dst, key)
+	return dst
+}
+
+// TODO(https://go.dev/issue/53693): Use hex.AppendEncode instead.
+func hexAppendEncode(dst, src []byte) []byte {
+	n := hex.EncodedLen(len(src))
+	dst = slices.Grow(dst, n)
+	hex.Encode(dst[len(dst):][:n], src)
+	return dst[:len(dst)+n]
 }
 
 // parseHex decodes a key string of the form "<prefix><hex string>"