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.

testing/ef_tests/src/handler.rs

@@ -52,7 +52,7 @@ pub trait Handler {
                 .filter(|e| e.file_type().map(|ty| ty.is_dir()).unwrap_or(false))
         };
         let test_cases = fs::read_dir(&handler_path)
-            .expect("handler dir exists")
+            .unwrap_or_else(|e| panic!("handler dir {} exists: {:?}", handler_path.display(), e))
             .filter_map(as_directory)
             .flat_map(|suite| fs::read_dir(suite.path()).expect("suite dir exists"))
             .filter_map(as_directory)