Remove equivocating validators from fork choice (#3371)

## Issue Addressed

Closes https://github.com/sigp/lighthouse/issues/3241
Closes https://github.com/sigp/lighthouse/issues/3242

## Proposed Changes

* [x] Implement logic to remove equivocating validators from fork choice per https://github.com/ethereum/consensus-specs/pull/2845
* [x] Update tests to v1.2.0-rc.1. The new test which exercises `equivocating_indices` is passing.
* [x] Pull in some SSZ abstractions from the `tree-states` branch that make implementing Vec-compatible encoding for types like `BTreeSet` and `BTreeMap`.
* [x] Implement schema upgrades and downgrades for the database (new schema version is V11).
* [x] Apply attester slashings from blocks to fork choice

## Additional Info

* This PR doesn't need the `BTreeMap` impl, but `tree-states` does, and I don't think there's any harm in keeping it. But I could also be convinced to drop it.

Blocked on #3322.

consensus/proto_array/src/fork_choice_test_definition.rs

@@ -6,6 +6,7 @@ mod votes;
 use crate::proto_array_fork_choice::{Block, ExecutionStatus, ProtoArrayForkChoice};
 use crate::InvalidationOperation;
 use serde_derive::{Deserialize, Serialize};
+use std::collections::BTreeSet;
 use types::{
     AttestationShufflingId, Checkpoint, Epoch, EthSpec, ExecutionBlockHash, Hash256,
     MainnetEthSpec, Slot,
@@ -88,6 +89,7 @@ impl ForkChoiceTestDefinition {
             ExecutionStatus::Optimistic(ExecutionBlockHash::zero()),
         )
         .expect("should create fork choice struct");
+        let equivocating_indices = BTreeSet::new();
 
         for (op_index, op) in self.operations.into_iter().enumerate() {
             match op.clone() {
@@ -103,6 +105,7 @@ impl ForkChoiceTestDefinition {
                             finalized_checkpoint,
                             &justified_state_balances,
                             Hash256::zero(),
+                            &equivocating_indices,
                             Slot::new(0),
                             &spec,
                         )
@@ -130,6 +133,7 @@ impl ForkChoiceTestDefinition {
                             finalized_checkpoint,
                             &justified_state_balances,
                             proposer_boost_root,
+                            &equivocating_indices,
                             Slot::new(0),
                             &spec,
                         )
@@ -154,6 +158,7 @@ impl ForkChoiceTestDefinition {
                         finalized_checkpoint,
                         &justified_state_balances,
                         Hash256::zero(),
+                        &equivocating_indices,
                         Slot::new(0),
                         &spec,
                     );