separated bench module into benchfn and benchzstd

it shall be possible to use benchfn
without any dependency on zstd.

programs/zstdcli.c

@@ -32,7 +32,7 @@
 #include <errno.h>    /* errno */
 #include "fileio.h"   /* stdinmark, stdoutmark, ZSTD_EXTENSION */
 #ifndef ZSTD_NOBENCH
-#  include "bench.h"  /* BMK_benchFiles */
+#  include "benchzstd.h"  /* BMK_benchFiles */
 #endif
 #ifndef ZSTD_NODICT
 #  include "dibio.h"  /* ZDICT_cover_params_t, DiB_trainFromFiles() */