Reorder field in the block

Miners should choose a transaction from the network based on its reward,
not as part of the mining variability.
That's why it's a bad idea to place the transaction as last block element.

Reorder the fields to make less room for harmful mining optimization.

blockchain.py

@@ -68,19 +68,19 @@ class Block:
 
     def from_bytes(block_raw):
         assert len(block_raw) == 292
-        transaction_raw = block_raw[144:292]
+        transaction_raw = block_raw[0:148]
         if transaction_raw == 148 * b"\0":
             transaction = None
         else:
             transaction = Transaction.from_bytes(transaction_raw)
         return Block(
-            nonce = int.from_bytes(block_raw[0:8], "big"),
-            timestamp = int.from_bytes(block_raw[8:16], "big"),
-            previous_hash = block_raw[16:48],
-            message = block_raw[48:80],
-            difficulty_sum = int.from_bytes(block_raw[80:112], "big"),
-            miner_pubkey = block_raw[112:144],
             transaction = transaction,
+            message = block_raw[148:180],
+            miner_pubkey = block_raw[180:212],
+            previous_hash = block_raw[212:244],
+            timestamp = int.from_bytes(block_raw[244:252], "big"),
+            difficulty_sum = int.from_bytes(block_raw[252:284], "big"),
+            nonce = int.from_bytes(block_raw[284:292], "big"),
             own_hash = hashlib.sha256(block_raw).digest(),
             balances = None,
             used_transaction_ids = None,
@@ -162,13 +162,14 @@ class Block:
             transaction = 148 * b"\0"
         else:
             transaction = self.transaction.get_transaction_raw()
-        return self.nonce.to_bytes(8, "big") + \
-            self.timestamp.to_bytes(8, "big") + \
-            self.previous_hash + \
+        return
+            transaction + \
             self.message + \
-            self.difficulty_sum.to_bytes(32, "big") + \
             self.miner_pubkey + \
-            transaction
+            self.previous_hash + \
+            self.timestamp.to_bytes(8, "big") + \
+            self.difficulty_sum.to_bytes(32, "big") + \
+            self.nonce.to_bytes(8, "big")
 
 class Blockchain:
     def __init__(self):