bitcoind: getrawblock helper.

It turns out using the verbose=false parameter, you can get the raw block
from getblock.  Do that.

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

bitcoin/block.c

@@ -0,0 +1,38 @@
+#include "bitcoin/block.h"
+#include "bitcoin/tx.h"
+#include <ccan/str/hex/hex.h>
+
+/* Encoding is <blockhdr> <varint-num-txs> <tx>... */
+struct bitcoin_block *bitcoin_block_from_hex(const tal_t *ctx,
+					     const char *hex, size_t hexlen)
+{
+	struct bitcoin_block *b;
+	u8 *linear_tx;
+	const u8 *p;
+	size_t len, i, num;
+
+	if (hexlen && hex[hexlen-1] == '\n')
+		hexlen--;
+
+	/* Set up the block for success. */
+	b = tal(ctx, struct bitcoin_block);
+
+	/* De-hex the array. */
+	len = hex_data_size(hexlen);
+	p = linear_tx = tal_arr(ctx, u8, len);
+	if (!hex_decode(hex, hexlen, linear_tx, len))
+		return tal_free(b);
+
+	pull(&p, &len, &b->hdr, sizeof(b->hdr));
+	num = pull_varint(&p, &len);
+	b->tx = tal_arr(b, struct bitcoin_tx *, num);
+	for (i = 0; i < num; i++)
+		b->tx[i] = pull_bitcoin_tx(b->tx, &p, &len);
+
+	/* We should end up not overrunning, nor have extra */
+	if (!p || len)
+		return tal_free(b);
+
+	tal_free(linear_tx);
+	return b;
+}