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/tests/block_verification.rs

@@ -68,13 +68,13 @@ fn chain_segment_blocks() -> Vec<SignedBeaconBlock<E>> {
 
 fn junk_signature() -> Signature {
     let kp = generate_deterministic_keypair(VALIDATOR_COUNT);
-    let message = &[42, 42];
-    Signature::new(message, &kp.sk)
+    let message = Hash256::from_slice(&[42; 32]);
+    kp.sk.sign(message)
 }
 
 fn junk_aggregate_signature() -> AggregateSignature {
-    let mut agg_sig = AggregateSignature::new();
-    agg_sig.add(&junk_signature());
+    let mut agg_sig = AggregateSignature::empty();
+    agg_sig.add_assign(&junk_signature());
     agg_sig
 }