bitcoin: Compute block hash while parsing

This avoids having to re-serialize the block header just to compute the
hash. It also frees us from having to carry around all the details in the
header and we can hand around a minimal version.

bitcoin/block.c

@@ -60,7 +60,7 @@ bitcoin_block_from_hex(const tal_t *ctx, const struct chainparams *chainparams,
 		b->hdr.nonce = pull_le32(&p, &len);
 		sha256_le32(&shactx, b->hdr.nonce);
 	}
-	sha256_double_done(&shactx, &b->hdr.hash);
+	sha256_double_done(&shactx, &b->hdr.hash.shad);
 
 	num = pull_varint(&p, &len);
 	b->tx = tal_arr(b, struct bitcoin_tx *, num);

bitcoin/block.h

@@ -23,7 +23,7 @@ struct bitcoin_block_hdr {
 	le32 timestamp;
 	le32 target;
 	le32 nonce;
-	struct sha256_double hash;
+	struct bitcoin_blkid hash;
 };
 
 struct elements_block_proof {