Support multiple BLS implementations (#1335)

## Issue Addressed

NA

## Proposed Changes

- Refactor the `bls` crate to support multiple BLS "backends" (e.g., milagro, blst, etc).
- Removes some duplicate, unused code in `common/rest_types/src/validator.rs`.
- Removes the old "upgrade legacy keypairs" functionality (these were unencrypted keys that haven't been supported for a few testnets, no one should be using them anymore).

## Additional Info

Most of the files changed are just inconsequential changes to function names.

## TODO

- [x] Optimization levels
- [x] Infinity point: https://github.com/supranational/blst/issues/11
- [x] Ensure milagro *and* blst are tested via CI
- [x] What to do with unsafe code?
- [x] Test infinity point in signature sets

beacon_node/beacon_chain/src/attestation_verification.rs

@@ -521,7 +521,7 @@ pub fn verify_attestation_signature<T: BeaconChainTypes>(
     let _signature_verification_timer =
         metrics::start_timer(&metrics::ATTESTATION_PROCESSING_SIGNATURE_TIMES);
 
-    if signature_set.is_valid() {
+    if signature_set.verify() {
         Ok(())
     } else {
         Err(Error::InvalidSignature)
@@ -589,7 +589,7 @@ pub fn verify_signed_aggregate_signatures<T: BeaconChainTypes>(
         .map_err(BeaconChainError::SignatureSetError)?,
     ];
 
-    Ok(verify_signature_sets(signature_sets))
+    Ok(verify_signature_sets(signature_sets.iter()))
 }
 
 /// Assists in readability.