common: pull out scriptPubkey address method

We're going to need this for P2WSH scripts. pull it out into
a common file plus adopt the sanity checks so that it will allow for
either P2WSH or P2WPKH (previously only encoded P2WPKH scripts)

common/addr.c

@@ -0,0 +1,22 @@
+#include "addr.h"
+#include <bitcoin/script.h>
+#include <common/bech32.h>
+
+/* Returns NULL if the script is not a P2WPKH or P2WSH */
+char *encode_scriptpubkey_to_addr(const tal_t *ctx,
+				   const char *hrp,
+				   const u8 *scriptPubkey)
+{
+	char *out;
+	size_t scriptLen = tal_bytelen(scriptPubkey);
+
+        /* Check that scriptPubkey is P2WSH or P2WPKH */
+	if (!is_p2wsh(scriptPubkey, NULL) && !is_p2wpkh(scriptPubkey, NULL))
+                return NULL;
+
+	out = tal_arr(ctx, char, 73 + strlen(hrp));
+	if (!segwit_addr_encode(out, hrp, 0, scriptPubkey + 2, scriptLen - 2))
+		return tal_free(out);
+
+	return out;
+}