Bulk signature verification (#507)

* Add basic block processing benches

* Start reviving state processing benches

* Fix old block builders

* Add optimization for faster pubkey add

* Tidy benches, add another

* Add extra block processing bench

* Start working on faster BLS scheme

* Add partially complete sig verify optimization

* Add .gitignore to state processing

* Add progress on faster signature verification

* Fix SignatureSet for fake_crypto

* Tidy attester slashings sig set

* Tidy bulk signature verifier

* Refactor signature sets to be cleaner

* Start threading SignatureStrategy through code

* Add (empty) test dir

* Move BenchingBlockBuilder

* Add initial block signature verification tests

* Add tests for bulk signature verification

* Start threading SignatureStrategy in block proc.

* Refactor per_block_processing errors

* Use sig set tuples instead of lists of two

* Remove dead code

* Thread VerifySignatures through per_block_processing

* Add bulk signature verification

* Introduce parallel bulk signature verification

* Expand state processing benches

* Fix additional compile errors

* Fix issue where par iter chunks is 0

* Update milagro_bls dep

* Remove debugs, code fragment in beacon chain

* Tidy, add comments to block sig verifier

* Fix various PR comments

* Add block_root option to per_block_processing

* Fix comment in block signature verifier

* Fix comments from PR review

* Remove old comment

* Fix comment

eth2/types/src/beacon_state.rs

@@ -398,6 +398,10 @@ impl<T: EthSpec> BeaconState<T> {
             .ok_or_else(|| Error::SlotOutOfBounds)?;
         let seed = self.get_seed(epoch, spec)?;
 
+        if first_committee.is_empty() {
+            return Err(Error::InsufficientValidators);
+        }
+
         let mut i = 0;
         Ok(loop {
             let candidate_index = first_committee[(epoch.as_usize() + i) % first_committee.len()];

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

@@ -119,6 +119,10 @@ impl<T: EthSpec> TestingBeaconBlockBuilder<T> {
         // - The shard of the committee.
         let mut committees: Vec<(Slot, Vec<usize>, Vec<usize>, u64)> = vec![];
 
+        if slot < T::slots_per_epoch() {
+            panic!("slot is too low, will get stuck in loop")
+        }
+
         // Loop backwards through slots gathering each committee, until:
         //
         // - The slot is too old to be included in a block at this slot.