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/operation_pool/src/lib.rs

@@ -901,8 +901,11 @@ mod release_tests {
         }
 
         fn attester_slashing(&self, slashed_indices: &[u64]) -> AttesterSlashing<MainnetEthSpec> {
-            let signer =
-                |idx: u64, message: &[u8]| Signature::new(message, &self.keypairs[idx as usize].sk);
+            let signer = |idx: u64, message: &[u8]| {
+                self.keypairs[idx as usize]
+                    .sk
+                    .sign(Hash256::from_slice(&message))
+            };
             TestingAttesterSlashingBuilder::double_vote(
                 AttesterSlashingTestTask::Valid,
                 slashed_indices,