Make `state_transition` not return post state; the given pre state should have been mutated to post state.

specs/phase0/beacon-chain.md

@@ -1202,7 +1202,7 @@ Let `genesis_block = BeaconBlock(state_root=hash_tree_root(genesis_state))`.
 The post-state corresponding to a pre-state `state` and a signed block `signed_block` is defined as `state_transition(state, signed_block)`. State transitions that trigger an unhandled exception (e.g. a failed `assert` or an out-of-range list access) are considered invalid. State transitions that cause a `uint64` overflow or underflow are also considered invalid.
 
 ```python
-def state_transition(state: BeaconState, signed_block: SignedBeaconBlock, validate_result: bool=True) -> BeaconState:
+def state_transition(state: BeaconState, signed_block: SignedBeaconBlock, validate_result: bool=True) -> None:
     block = signed_block.message
     # Process slots (including those with no blocks) since block
     process_slots(state, block.slot)
@@ -1214,8 +1214,6 @@ def state_transition(state: BeaconState, signed_block: SignedBeaconBlock, valida
     # Verify state root
     if validate_result:
         assert block.state_root == hash_tree_root(state)
-    # Return post-state
-    return state
 ```
 
 ```python

specs/phase0/fork-choice.md

@@ -355,7 +355,8 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     assert get_ancestor(store, block.parent_root, finalized_slot) == store.finalized_checkpoint.root
 
     # Check the block is valid and compute the post-state
-    state = state_transition(pre_state, signed_block, True)
+    state = pre_state.copy()
+    state_transition(state, signed_block, True)
     # Add new block to the store
     store.blocks[hash_tree_root(block)] = block
     # Add new state for this block to the store

specs/phase0/validator.md

@@ -372,7 +372,7 @@ It is useful to be able to run a state transition function (working on a copy of
 def compute_new_state_root(state: BeaconState, block: BeaconBlock) -> Root:
     temp_state: BeaconState = state.copy()
     signed_block = SignedBeaconBlock(message=block)
-    temp_state = state_transition(temp_state, signed_block, validate_result=False)
+    state_transition(temp_state, signed_block, validate_result=False)
     return hash_tree_root(temp_state)
 ```