Update VC and BN APIs for naive aggregation (#950)

* Refactor `Attestation` production

* Add constant

* Start refactor for aggregation

* Return early when no attesting validators

* Refactor into individual functions

* Tidy, add comments

* Add first draft of NaiveAggregationPool

* Further progress on naive aggregation pool

* Fix compile errors in VC

* Change locking logic for naive pool

* Introduce AttesationType

* Add pruning, comments

* Add MAX_ATTESTATIONS_PER_SLOT restriction

* Add pruning based on slot

* Update BN for new aggregation fns

* Fix test compile errors

* Fix failing rest_api test

* Move SignedAggregateAndProof into own file

* Update docs, fix warning

* Tidy some formatting in validator API

* Remove T::default_spec from signing

* Fix failing rest test

* Tidy

* Add test, fix bug

* Improve naive pool tests

* Add max attestations test

* Revert changes to the op_pool

* Refactor timer

beacon_node/beacon_chain/src/builder.rs

@@ -415,6 +415,8 @@ where
             op_pool: self
                 .op_pool
                 .ok_or_else(|| "Cannot build without op pool".to_string())?,
+            // TODO: allow for persisting and loading the pool from disk.
+            naive_aggregation_pool: <_>::default(),
             eth1_chain: self.eth1_chain,
             canonical_head: TimeoutRwLock::new(canonical_head.clone()),
             genesis_block_root: self