Start moving state_transition.py to state transitition spec

The state transition spec should be reasonably self-contained, limiting the amount of "magic" outside of it. This PR is a first step in this direction, specifically for operation processing.

specs/core/0_beacon-chain.md

@@ -1691,12 +1691,28 @@ def process_eth1_data(state: BeaconState, block: BeaconBlock) -> None:
 
 #### Operations
 
+The sub-sections below define helper functions, one per operation type. The full processing of operations is done by running `process_operations(state, block.body)`.
+
+```python
+def process_operations(state: BeaconState, body: BeaconBlockBody) -> None:
+    assert len(body.deposits) == min(MAX_DEPOSITS, state.latest_eth1_data.deposit_count - state.deposit_index)
+    assert len(body.transfers) == len(set(body.transfers))
+
+    for operations, max_operations, function in {
+        (body.proposer_slashings, MAX_PROPOSER_SLASHINGS, process_proposer_slashing),
+        (body.attester_slashings, MAX_ATTESTER_SLASHINGS, attester_slashings),
+        (body.attestations, MAX_ATTESTATIONS, process_attestation),
+        (body.deposits, MAX_DEPOSITS, process_deposit),
+        (body.voluntary_exits, MAX_VOLUNTARY_EXITS, process_voluntary_exit),
+        (body.transfers, MAX_TRANSFERS, process_transfer),
+    }:
+        assert len(operations) <= max_operations
+        for operation in operations:
+            function(state, operation)
+```
+
 ##### Proposer slashings
 
-Verify that `len(block.body.proposer_slashings) <= MAX_PROPOSER_SLASHINGS`.
-
-For each `proposer_slashing` in `block.body.proposer_slashings`, run the following function:
-
 ```python
 def process_proposer_slashing(state: BeaconState,
                               proposer_slashing: ProposerSlashing) -> None:
@@ -1721,10 +1737,6 @@ def process_proposer_slashing(state: BeaconState,
 
 ##### Attester slashings
 
-Verify that `len(block.body.attester_slashings) <= MAX_ATTESTER_SLASHINGS`.
-
-For each `attester_slashing` in `block.body.attester_slashings`, run the following function:
-
 ```python
 def process_attester_slashing(state: BeaconState,
                               attester_slashing: AttesterSlashing) -> None:
@@ -1759,10 +1771,6 @@ def process_attester_slashing(state: BeaconState,
 
 ##### Attestations
 
-Verify that `len(block.body.attestations) <= MAX_ATTESTATIONS`.
-
-For each `attestation` in `block.body.attestations`, run the following function:
-
 ```python
 def process_attestation(state: BeaconState, attestation: Attestation) -> None:
     """
@@ -1801,10 +1809,6 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
 
 ##### Deposits
 
-Verify that `len(block.body.deposits) == min(MAX_DEPOSITS, state.latest_eth1_data.deposit_count - state.deposit_index)`.
-
-For each `deposit` in `block.body.deposits`, run the following function:
-
 ```python
 def process_deposit(state: BeaconState, deposit: Deposit) -> None:
     """
@@ -1851,10 +1855,6 @@ def process_deposit(state: BeaconState, deposit: Deposit) -> None:
 
 ##### Voluntary exits
 
-Verify that `len(block.body.voluntary_exits) <= MAX_VOLUNTARY_EXITS`.
-
-For each `exit` in `block.body.voluntary_exits`, run the following function:
-
 ```python
 def process_voluntary_exit(state: BeaconState, exit: VoluntaryExit) -> None:
     """
@@ -1879,10 +1879,6 @@ def process_voluntary_exit(state: BeaconState, exit: VoluntaryExit) -> None:
 
 ##### Transfers
 
-Verify that `len(block.body.transfers) <= MAX_TRANSFERS` and that all transfers are distinct.
-
-For each `transfer` in `block.body.transfers`, run the following function:
-
 ```python
 def process_transfer(state: BeaconState, transfer: Transfer) -> None:
     """