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/Cargo.toml

@@ -14,7 +14,8 @@ eth2_ssz_derive = "0.3.0"
 
 [dependencies]
 ethereum-types = "0.12.1"
-smallvec = "1.6.1"
+smallvec = { version = "1.6.1", features = ["const_generics"] }
+itertools = "0.10.3"
 
 [features]
 arbitrary = ["ethereum-types/arbitrary"]