common: add helpers for bitcoin blockids.

diff --git a/bitcoin/block.c b/bitcoin/block.c
index 5a36a7df..6838f2c3 100644
--- a/bitcoin/block.c
+++ b/bitcoin/block.c
@@ -228,16 +228,24 @@ void bitcoin_block_blkid(const struct bitcoin_block *b,
 	*out = b->hdr.hash;
 }
 
-static bool bitcoin_blkid_to_hex(const struct bitcoin_blkid *blockid,
-				 char *hexstr, size_t hexstr_len)
+bool bitcoin_blkid_from_hex(const char *hexstr, size_t hexstr_len,
+			   struct bitcoin_blkid *blkid)
 {
-	struct bitcoin_txid fake_txid;
-	fake_txid.shad = blockid->shad;
-	return bitcoin_txid_to_hex(&fake_txid, hexstr, hexstr_len);
+	if (!hex_decode(hexstr, hexstr_len, blkid, sizeof(*blkid)))
+		return false;
+	reverse_bytes(blkid->shad.sha.u.u8, sizeof(blkid->shad.sha.u.u8));
+	return true;
 }
 
-char *fmt_bitcoin_blkid(const tal_t *ctx,
-			const struct bitcoin_blkid *blkid)
+bool bitcoin_blkid_to_hex(const struct bitcoin_blkid *blkid,
+			 char *hexstr, size_t hexstr_len)
+{
+	struct sha256_double rev = blkid->shad;
+	reverse_bytes(rev.sha.u.u8, sizeof(rev.sha.u.u8));
+	return hex_encode(&rev, sizeof(rev), hexstr, hexstr_len);
+}
+
+char *fmt_bitcoin_blkid(const tal_t *ctx, const struct bitcoin_blkid *blkid)
 {
 	char *hexstr = tal_arr(ctx, char, hex_str_size(sizeof(*blkid)));
 
diff --git a/bitcoin/block.h b/bitcoin/block.h
index a3289ee7..8a945ca4 100644
--- a/bitcoin/block.h
+++ b/bitcoin/block.h
@@ -52,6 +52,11 @@ void fromwire_chainparams(const u8 **cursor, size_t *max,
 			  const struct chainparams **chainparams);
 void towire_chainparams(u8 **cursor, const struct chainparams *chainparams);
 
+bool bitcoin_blkid_from_hex(const char *hexstr, size_t hexstr_len,
+			    struct bitcoin_blkid *blkid);
+bool bitcoin_blkid_to_hex(const struct bitcoin_blkid *blkid,
+			  char *hexstr, size_t hexstr_len);
+
 char *fmt_bitcoin_blkid(const tal_t *ctx,
 			const struct bitcoin_blkid *blkid);
 
diff --git a/bitcoin/test/run-bitcoin_block_from_hex.c b/bitcoin/test/run-bitcoin_block_from_hex.c
index 0dc0bd64..94b26ad5 100644
--- a/bitcoin/test/run-bitcoin_block_from_hex.c
+++ b/bitcoin/test/run-bitcoin_block_from_hex.c
@@ -62,15 +62,6 @@ static const char block[] =
 
 STRUCTEQ_DEF(sha256_double, 0, sha);
 
-static bool bitcoin_blkid_from_hex(const char *hexstr, size_t hexstr_len,
-				   struct bitcoin_blkid *blockid)
-{
-	struct bitcoin_txid fake_txid;
-	if (!bitcoin_txid_from_hex(hexstr, hexstr_len, &fake_txid))
-		return false;
-	blockid->shad = fake_txid.shad;
-	return true;
-}
 int main(int argc, const char *argv[])
 {
 	struct bitcoin_blkid prev;
diff --git a/common/json_parse.c b/common/json_parse.c
index 7841e082..5c1c6831 100644
--- a/common/json_parse.c
+++ b/common/json_parse.c
@@ -601,6 +601,13 @@ bool json_to_txid(const char *buffer, const jsmntok_t *tok,
 				     tok->end - tok->start, txid);
 }
 
+bool json_to_bitcoin_blkid(const char *buffer, const jsmntok_t *tok,
+			   struct bitcoin_blkid *blkid)
+{
+	return bitcoin_blkid_from_hex(buffer + tok->start,
+				      tok->end - tok->start, blkid);
+}
+
 bool json_to_outpoint(const char *buffer, const jsmntok_t *tok,
 		      struct bitcoin_outpoint *op)
 {
diff --git a/common/json_parse.h b/common/json_parse.h
index 4706c377..4c739154 100644
--- a/common/json_parse.h
+++ b/common/json_parse.h
@@ -108,6 +108,10 @@ bool json_to_msat(const char *buffer, const jsmntok_t *tok,
 bool json_to_txid(const char *buffer, const jsmntok_t *tok,
 		  struct bitcoin_txid *txid);
 
+/* Extract a bitcoin blkid from this */
+bool json_to_bitcoin_blkid(const char *buffer, const jsmntok_t *tok,
+			   struct bitcoin_blkid *blkid);
+
 /* Extract a bitcoin outpoint from this */
 bool json_to_outpoint(const char *buffer, const jsmntok_t *tok,
 		      struct bitcoin_outpoint *op);
diff --git a/common/json_stream.c b/common/json_stream.c
index 6a074607..3daefe46 100644
--- a/common/json_stream.c
+++ b/common/json_stream.c
@@ -455,6 +455,15 @@ void json_add_txid(struct json_stream *result, const char *fieldname,
 	json_add_string(result, fieldname, hex);
 }
 
+void json_add_bitcoin_blkid(struct json_stream *result, const char *fieldname,
+			    const struct bitcoin_blkid *blkid)
+{
+	char hex[hex_str_size(sizeof(*blkid))];
+
+	bitcoin_blkid_to_hex(blkid, hex, sizeof(hex));
+	json_add_string(result, fieldname, hex);
+}
+
 void json_add_outpoint(struct json_stream *result, const char *fieldname,
 		       const struct bitcoin_outpoint *out)
 {
diff --git a/common/json_stream.h b/common/json_stream.h
index 7756c013..3263dfd9 100644
--- a/common/json_stream.h
+++ b/common/json_stream.h
@@ -31,6 +31,7 @@ struct short_channel_id;
 struct sha256;
 struct preimage;
 struct bitcoin_tx;
+struct bitcoin_blkid;
 struct wally_psbt;
 struct lease_rates;
 struct wireaddr;
@@ -310,6 +311,10 @@ void json_add_channel_id(struct json_stream *response,
 void json_add_txid(struct json_stream *result, const char *fieldname,
 		   const struct bitcoin_txid *txid);
 
+/* '"fieldname" : <hexrev>' or "<hexrev>" if fieldname is NULL */
+void json_add_bitcoin_blkid(struct json_stream *result, const char *fieldname,
+			    const struct bitcoin_blkid *blkid);
+
 /* '"fieldname" : "txid:n" */
 void json_add_outpoint(struct json_stream *result, const char *fieldname,
 		       const struct bitcoin_outpoint *out);