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/eth1/tests/test.rs

@@ -44,7 +44,7 @@ fn random_deposit_data() -> DepositData {
         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, &MainnetEthSpec::default_spec());