Implement "gocryptfs -speed"

A crypto benchmark mode like "openssl speed".

Example run:

  $ ./gocryptfs -speed
  AES-GCM-256-OpenSSL 	 180.89 MB/s	(selected in auto mode)
  AES-GCM-256-Go      	  48.19 MB/s
  AES-SIV-512-Go      	  37.40 MB/s

1 files changed, 0 insertions, 58 deletions

diff --git a/internal/stupidgcm/stupidgcm_test.go b/internal/stupidgcm/stupidgcm_test.go
index d6c0714..ba25855 100644
--- a/internal/stupidgcm/stupidgcm_test.go
+++ b/internal/stupidgcm/stupidgcm_test.go
@@ -15,9 +15,6 @@ import (
 	"encoding/hex"
 	"log"
 	"testing"
-
-	// For benchmark comparison
-	"github.com/rfjakob/gocryptfs/internal/siv_aead"
 )
 
 // Get "n" random bytes from /dev/urandom or panic
@@ -121,58 +118,3 @@ func TestCorruption(t *testing.T) {
 		t.Fatalf("Should have gotten error")
 	}
 }
-
-// $ go test -bench .
-// PASS
-// Benchmark4kEncStupidGCM-2	   50000	     25622 ns/op	 159.86 MB/s
-// Benchmark4kEncGoGCM-2    	   10000	    116544 ns/op	  35.15 MB/s
-// ok  	github.com/rfjakob/gocryptfs/internal/stupidgcm	3.775s
-func Benchmark4kEncStupidGCM(b *testing.B) {
-	key := randBytes(32)
-	authData := randBytes(24)
-	iv := randBytes(16)
-	in := make([]byte, 4096)
-	b.SetBytes(int64(len(in)))
-
-	sGCM := New(key)
-
-	for i := 0; i < b.N; i++ {
-		// Encrypt and append to nonce
-		sGCM.Seal(iv, iv, in, authData)
-	}
-}
-
-func Benchmark4kEncGoGCM(b *testing.B) {
-	key := randBytes(32)
-	authData := randBytes(24)
-	iv := randBytes(16)
-	in := make([]byte, 4096)
-	b.SetBytes(int64(len(in)))
-
-	gAES, err := aes.NewCipher(key)
-	if err != nil {
-		b.Fatal(err)
-	}
-	gGCM, err := cipher.NewGCMWithNonceSize(gAES, 16)
-	if err != nil {
-		b.Fatal(err)
-	}
-
-	for i := 0; i < b.N; i++ {
-		// Encrypt and append to nonce
-		gGCM.Seal(iv, iv, in, authData)
-	}
-}
-
-func Benchmark4kEncAESSIV(b *testing.B) {
-	key := randBytes(32)
-	authData := randBytes(24)
-	iv := randBytes(16)
-	in := make([]byte, 4096)
-	b.SetBytes(int64(len(in)))
-	gGCM := siv_aead.New(key)
-	for i := 0; i < b.N; i++ {
-		// Encrypt and append to nonce
-		gGCM.Seal(iv, iv, in, authData)
-	}
-}