Revert "Flags to BitVector"

This reverts commit 786e611c71.

# Conflicts:
#	specs/altair/beacon-chain.md

specs/altair/beacon-chain.md

@@ -27,6 +27,9 @@
 - [Helper functions](#helper-functions)
   - [`Predicates`](#predicates)
     - [`eth2_fast_aggregate_verify`](#eth2_fast_aggregate_verify)
+  - [Misc](#misc-2)
+    - [`add_flag`](#add_flag)
+    - [`has_flag`](#has_flag)
   - [Beacon state accessors](#beacon-state-accessors)
     - [`get_sync_committee_indices`](#get_sync_committee_indices)
     - [`get_sync_committee`](#get_sync_committee)
@@ -65,7 +68,7 @@ Altair is the first beacon chain hard fork. Its main features are:
 
 | Name | SSZ equivalent | Description |
 | - | - | - |
-| `ParticipationFlags` | `BitVector[PARTICIPATION_FLAGS_LENGTH]` | a succinct representation of up to 8 boolean participation flags |
+| `ParticipationFlags` | `uint8` | a succinct representation of 8 boolean participation flags |
 
 ## Constants
 
@@ -95,7 +98,6 @@ Altair is the first beacon chain hard fork. Its main features are:
 | Name | Value |
 | - | - |
 | `G2_POINT_AT_INFINITY` | `BLSSignature(b'\xc0' + b'\x00' * 95)` |
-| `PARTICIPATION_FLAGS_LENGTH` | `8` |
 | `PARTICIPATION_FLAGS` | `3` |
 | `PARTICIPATION_FLAG_WEIGHTS` | `[TIMELY_HEAD_WEIGHT, TIMELY_SOURCE_WEIGHT, TIMELY_TARGET_WEIGHT]` |
 
@@ -231,6 +233,30 @@ def eth2_fast_aggregate_verify(pubkeys: Sequence[BLSPubkey], message: Bytes32, s
     return bls.FastAggregateVerify(pubkeys, message, signature)
 ```
 
+### Misc
+
+#### `add_flag`
+
+```python
+def add_flag(flags: ParticipationFlags, flag_index: int) -> ParticipationFlags:
+    """
+    Return a new ``ParticipationFlags`` adding ``flag_index`` to ``flags``.
+    """
+    flag = ParticipationFlags(2**flag_index)
+    return flags | flag
+```
+
+#### `has_flag`
+
+```python
+def has_flag(flags: ParticipationFlags, flag_index: int) -> bool:
+    """
+    Return whether ``flags`` has ``flag_index`` set.
+    """
+    flag = ParticipationFlags(2**flag_index)
+    return flags & flag == flag
+```
+
 ### Beacon state accessors
 
 #### `get_sync_committee_indices`
@@ -311,7 +337,7 @@ def get_unslashed_participating_indices(state: BeaconState, flag_index: int, epo
     else:
         epoch_participation = state.previous_epoch_participation
     active_validator_indices = get_active_validator_indices(state, epoch)
-    participating_indices = [i for i in active_validator_indices if epoch_participation[i][flag_index]]
+    participating_indices = [i for i in active_validator_indices if has_flag(epoch_participation[i], flag_index)]
     return set(filter(lambda index: not state.validators[index].slashed, participating_indices))
 ```