Add attester/proposer slashing endpoints (#856)

* Remove deprecated api_spec.yaml

* add prototype for proposer slashing

* remove clippy warnings

* Add proposer_slashing API

* Prototype for attester slashing API call

* Fix logic error in operation pool

* Finish test for attester_slashing api call

* Clean proposer_slashing test

* Cargo fmt

* Remove useless to_string after format! macro

* Cargo fmt

* Update book with new api calls

* Re-enable proposer slashing verification

* Update book with appropriate test example

* Fix proposer_slashing test

* Update comments and tests for clearer code

* Remove extraneous comments

* Fix test

* Minor fix

* Address reviewer comments

Co-authored-by: pscott <30843220+pscott@users.noreply.github.com>

eth2/types/src/test_utils/builders/testing_beacon_block_builder.rs

@@ -377,7 +377,7 @@ impl<T: EthSpec> TestingBeaconBlockBuilder<T> {
 /// Builds an `ProposerSlashing` for some `validator_index`.
 ///
 /// Signs the message using a `BeaconChainHarness`.
-fn build_proposer_slashing<T: EthSpec>(
+pub fn build_proposer_slashing<T: EthSpec>(
     test_task: ProposerSlashingTestTask,
     validator_index: u64,
     secret_key: &SecretKey,
@@ -396,7 +396,7 @@ fn build_proposer_slashing<T: EthSpec>(
 /// Builds an `AttesterSlashing` for some `validator_indices`.
 ///
 /// Signs the message using a `BeaconChainHarness`.
-fn build_double_vote_attester_slashing<T: EthSpec>(
+pub fn build_double_vote_attester_slashing<T: EthSpec>(
     test_task: AttesterSlashingTestTask,
     validator_indices: &[u64],
     secret_keys: &[&SecretKey],