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

validator_client/src/validator_store.rs

@@ -129,7 +129,7 @@ impl<T: SlotClock + 'static, E: EthSpec> ValidatorStore<T, E> {
                 );
                 let message = epoch.signing_root(domain);
 
-                Some(Signature::new(message.as_bytes(), &voting_keypair.sk))
+                Some(voting_keypair.sk.sign(message))
             })
     }