stupidgcm: NewChacha20poly1305: avoid slice append

I noticed that growslice() shows up in the cpuprofile.
Avoiding slice append for the private jey copy gives a 0.6% speedup:

gocryptfs/internal/speed$ benchstat old new
name             old time/op   new time/op   delta
StupidXchacha-4   5.68µs ± 0%   5.65µs ± 0%  -0.63%  (p=0.008 n=5+5)

name             old speed     new speed     delta
StupidXchacha-4  721MB/s ± 0%  725MB/s ± 0%  +0.63%  (p=0.008 n=5+5)

internal/stupidgcm/chacha.go

@@ -39,9 +39,12 @@ func NewChacha20poly1305(key []byte) *stupidChacha20poly1305 {
 	if len(key) != chacha20poly1305.KeySize {
 		log.Panicf("Only %d-byte keys are supported, you passed %d bytes", chacha20poly1305.KeySize, len(key))
 	}
+	// private copy
+	key2 := make([]byte, chacha20poly1305.KeySize)
+	copy(key2, key)
 	return &stupidChacha20poly1305{
 		stupidAEADCommon{
-			key:              append([]byte{}, key...), // private copy
+			key:              key2,
 			openSSLEVPCipher: _EVP_chacha20_poly1305,
 			nonceSize:        chacha20poly1305.NonceSize,
 		},