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/errors.rs

@@ -1,5 +1,6 @@
 use crate::eth1_chain::Error as Eth1ChainError;
 use crate::fork_choice::Error as ForkChoiceError;
+use crate::naive_aggregation_pool::Error as NaiveAggregationError;
 use operation_pool::OpPoolError;
 use ssz::DecodeError;
 use ssz_types::Error as SszTypesError;
@@ -64,6 +65,7 @@ pub enum BeaconChainError {
     DuplicateValidatorPublicKey,
     ValidatorPubkeyCacheFileError(String),
     OpPoolError(OpPoolError),
+    NaiveAggregationError(NaiveAggregationError),
 }
 
 easy_from_to!(SlotProcessingError, BeaconChainError);
@@ -71,6 +73,7 @@ easy_from_to!(AttestationValidationError, BeaconChainError);
 easy_from_to!(SszTypesError, BeaconChainError);
 easy_from_to!(OpPoolError, BeaconChainError);
 easy_from_to!(BlockSignatureVerifierError, BeaconChainError);
+easy_from_to!(NaiveAggregationError, BeaconChainError);
 
 #[derive(Debug, PartialEq)]
 pub enum BlockProductionError {