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, 6 insertions, 0 deletions

diff --git a/main.go b/main.go
index 21f2d06..9301ae1 100644
--- a/main.go
+++ b/main.go
@@ -13,6 +13,7 @@ import (
 	"github.com/rfjakob/gocryptfs/internal/configfile"
 	"github.com/rfjakob/gocryptfs/internal/contentenc"
 	"github.com/rfjakob/gocryptfs/internal/readpassword"
+	"github.com/rfjakob/gocryptfs/internal/speed"
 	"github.com/rfjakob/gocryptfs/internal/stupidgcm"
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
@@ -149,6 +150,11 @@ func main() {
 		printVersion()
 		os.Exit(0)
 	}
+	// "-speed"
+	if args.speed {
+		speed.Run()
+		os.Exit(0)
+	}
 	if args.wpanic {
 		tlog.Warn.Wpanic = true
 		tlog.Debug.Printf("Panicing on warnings")