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/op_verification.rs

@@ -201,7 +201,11 @@ fn attester_slashing() {
     // Last half of the validators
     let second_half = (VALIDATOR_COUNT as u64 / 2..VALIDATOR_COUNT as u64).collect::<Vec<_>>();
 
-    let signer = |idx: u64, message: &[u8]| Signature::new(message, &KEYPAIRS[idx as usize].sk);
+    let signer = |idx: u64, message: &[u8]| {
+        KEYPAIRS[idx as usize]
+            .sk
+            .sign(Hash256::from_slice(&message))
+    };
 
     let make_slashing = |validators| {
         TestingAttesterSlashingBuilder::double_vote::<_, E>(