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/errors.rs

@@ -6,7 +6,6 @@ use crate::observed_attesters::Error as ObservedAttestersError;
 use crate::observed_block_producers::Error as ObservedBlockProducersError;
 use operation_pool::OpPoolError;
 use safe_arith::ArithError;
-use ssz::DecodeError;
 use ssz_types::Error as SszTypesError;
 use state_processing::{
     block_signature_verifier::Error as BlockSignatureVerifierError,
@@ -69,7 +68,7 @@ pub enum BeaconChainError {
     AttestationCacheLockTimeout,
     ValidatorPubkeyCacheLockTimeout,
     IncorrectStateForAttestation(RelativeEpochError),
-    InvalidValidatorPubkeyBytes(DecodeError),
+    InvalidValidatorPubkeyBytes(bls::Error),
     ValidatorPubkeyCacheIncomplete(usize),
     SignatureSetError(SignatureSetError),
     BlockSignatureVerifierError(state_processing::block_signature_verifier::Error),