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, 29 insertions, 0 deletions

diff --git a/internal/speed/speed_test.go b/internal/speed/speed_test.go
new file mode 100644
index 0000000..1e9d859
--- /dev/null
+++ b/internal/speed/speed_test.go
@@ -0,0 +1,29 @@
+package speed
+
+/*
+Make the "-speed" benchmarks also accessible to the standard test system.
+Example run:
+
+$ go test -bench .
+BenchmarkStupidGCM-2   	  100000	     22552 ns/op	 181.62 MB/s
+BenchmarkGoGCM-2       	   20000	     81871 ns/op	  50.03 MB/s
+BenchmarkAESSIV-2      	   10000	    104623 ns/op	  39.15 MB/s
+PASS
+ok  	github.com/rfjakob/gocryptfs/internal/speed	6.022s
+*/
+
+import (
+	"testing"
+)
+
+func BenchmarkStupidGCM(b *testing.B) {
+	bStupidGCM(b)
+}
+
+func BenchmarkGoGCM(b *testing.B) {
+	bGoGCM(b)
+}
+
+func BenchmarkAESSIV(b *testing.B) {
+	bAESSIV(b)
+}