devtools/features: tool to convert feature bitmap to names.

For example:

```
$ ./devtools/features 80008008226aa2
option_data_loss_protect/odd (optional)
option_upfront_shutdown_script/odd (optional)
option_gossip_queries/odd (optional)
option_var_onion_optin/odd (optional)
option_gossip_queries_ex/odd (optional)
option_static_remotekey/odd (optional)
option_payment_secret/even (compulsory)
option_basic_mpp/odd (optional)
option_anchor_outputs/odd (optional)
option_shutdown_anysegwit/odd (optional)
option_onion_messages/odd (optional)
option_unknown_54/odd (optional)
```


Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

devtools/features.c

@@ -0,0 +1,28 @@
+/* Turns a hex string into feature. */
+#include "config.h"
+#include <ccan/err/err.h>
+#include <common/features.h>
+#include <common/utils.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[])
+{
+	const u8 *features;
+
+	setup_locale();
+
+	if (argc != 2)
+		errx(1, "Usage: %s <hexstring>", argv[0]);
+
+	features = tal_hexdata(NULL, argv[1], strlen(argv[1]));
+	if (!features)
+		errx(1, "bad hexstring");
+
+	for (size_t i = 0; i < tal_bytelen(features) * 8; i++) {
+		if (feature_is_set(features, i))
+			printf("%s (%s)\n",
+			       feature_name(features, i),
+			       i % 2 ? "optional" : "compulsory");
+	}
+	tal_free(features);
+}