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

testing/eth1_test_rig/src/lib.rs

@@ -124,7 +124,7 @@ impl DepositContract {
             pubkey: keypair.pk.into(),
             withdrawal_credentials,
             amount,
-            signature: Signature::empty_signature().into(),
+            signature: Signature::empty().into(),
         };
 
         deposit.signature = deposit.create_signature(&keypair.sk, &E::default_spec());
@@ -142,7 +142,7 @@ impl DepositContract {
             pubkey: keypair.pk.into(),
             withdrawal_credentials: Hash256::zero(),
             amount: 32_000_000_000,
-            signature: Signature::empty_signature().into(),
+            signature: Signature::empty().into(),
         };
 
         deposit.signature = deposit.create_signature(&keypair.sk, &E::default_spec());
@@ -168,7 +168,7 @@ impl DepositContract {
             pubkey: keypair.pk.into(),
             withdrawal_credentials: Hash256::zero(),
             amount,
-            signature: Signature::empty_signature().into(),
+            signature: Signature::empty().into(),
         };
 
         deposit.signature = deposit.create_signature(&keypair.sk, &E::default_spec());