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/per_epoch_processing/resets.rs

@@ -0,0 +1,38 @@
+use super::errors::EpochProcessingError;
+use core::result::Result;
+use core::result::Result::Ok;
+use safe_arith::SafeArith;
+use types::beacon_state::BeaconState;
+use types::eth_spec::EthSpec;
+use types::{Unsigned, VariableList};
+
+pub fn process_eth1_data_reset<T: EthSpec>(
+    state: &mut BeaconState<T>,
+) -> Result<(), EpochProcessingError> {
+    if state
+        .slot()
+        .safe_add(1)?
+        .safe_rem(T::SlotsPerEth1VotingPeriod::to_u64())?
+        == 0
+    {
+        *state.eth1_data_votes_mut() = VariableList::empty();
+    }
+    Ok(())
+}
+
+pub fn process_slashings_reset<T: EthSpec>(
+    state: &mut BeaconState<T>,
+) -> Result<(), EpochProcessingError> {
+    let next_epoch = state.next_epoch()?;
+    state.set_slashings(next_epoch, 0)?;
+    Ok(())
+}
+
+pub fn process_randao_mixes_reset<T: EthSpec>(
+    state: &mut BeaconState<T>,
+) -> Result<(), EpochProcessingError> {
+    let current_epoch = state.current_epoch();
+    let next_epoch = state.next_epoch()?;
+    state.set_randao_mix(next_epoch, *state.get_randao_mix(current_epoch)?)?;
+    Ok(())
+}