Altair consensus changes and refactors (#2279)

## Proposed Changes

Implement the consensus changes necessary for the upcoming Altair hard fork.

## Additional Info

This is quite a heavy refactor, with pivotal types like the `BeaconState` and `BeaconBlock` changing from structs to enums. This ripples through the whole codebase with field accesses changing to methods, e.g. `state.slot` => `state.slot()`.


Co-authored-by: realbigsean <seananderson33@gmail.com>

consensus/state_processing/src/common/get_attestation_participation.rs

@@ -0,0 +1,54 @@
+use integer_sqrt::IntegerSquareRoot;
+use smallvec::SmallVec;
+use types::{
+    consts::altair::{
+        NUM_FLAG_INDICES, TIMELY_HEAD_FLAG_INDEX, TIMELY_SOURCE_FLAG_INDEX,
+        TIMELY_TARGET_FLAG_INDEX,
+    },
+    BeaconStateError as Error,
+};
+use types::{AttestationData, BeaconState, ChainSpec, EthSpec};
+
+/// Get the participation flags for a valid attestation.
+///
+/// You should have called `verify_attestation_for_block_inclusion` or similar before
+/// calling this function, in order to ensure that the attestation's source is correct.
+///
+/// This function will return an error if the source of the attestation doesn't match the
+/// state's relevant justified checkpoint.
+pub fn get_attestation_participation_flag_indices<T: EthSpec>(
+    state: &BeaconState<T>,
+    data: &AttestationData,
+    inclusion_delay: u64,
+    spec: &ChainSpec,
+) -> Result<SmallVec<[usize; NUM_FLAG_INDICES]>, Error> {
+    let justified_checkpoint = if data.target.epoch == state.current_epoch() {
+        state.current_justified_checkpoint()
+    } else {
+        state.previous_justified_checkpoint()
+    };
+
+    // Matching roots.
+    let is_matching_source = data.source == justified_checkpoint;
+    let is_matching_target = is_matching_source
+        && data.target.root == *state.get_block_root_at_epoch(data.target.epoch)?;
+    let is_matching_head =
+        is_matching_target && data.beacon_block_root == *state.get_block_root(data.slot)?;
+
+    if !is_matching_source {
+        return Err(Error::IncorrectAttestationSource);
+    }
+
+    // Participation flag indices
+    let mut participation_flag_indices = SmallVec::new();
+    if is_matching_source && inclusion_delay <= T::slots_per_epoch().integer_sqrt() {
+        participation_flag_indices.push(TIMELY_SOURCE_FLAG_INDEX);
+    }
+    if is_matching_target && inclusion_delay <= T::slots_per_epoch() {
+        participation_flag_indices.push(TIMELY_TARGET_FLAG_INDEX);
+    }
+    if is_matching_head && inclusion_delay == spec.min_attestation_inclusion_delay {
+        participation_flag_indices.push(TIMELY_HEAD_FLAG_INDEX);
+    }
+    Ok(participation_flag_indices)
+}