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/ssz/src/decode.rs

@@ -5,6 +5,7 @@ use std::cmp::Ordering;
 type SmallVec8<T> = SmallVec<[T; 8]>;
 
 pub mod impls;
+pub mod try_from_iter;
 
 /// Returned when SSZ decoding fails.
 #[derive(Debug, PartialEq, Clone)]