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

crypto/eth2_keystore/tests/eip2335_vectors.rs

@@ -14,7 +14,7 @@ pub fn decode_and_check_sk(json: &str) -> Keystore {
     let keystore = Keystore::from_json_str(json).expect("should decode keystore json");
     let expected_sk = hex::decode(EXPECTED_SECRET).unwrap();
     let keypair = keystore.decrypt_keypair(PASSWORD.as_bytes()).unwrap();
-    assert_eq!(keypair.sk.as_bytes().as_ref(), &expected_sk[..]);
+    assert_eq!(keypair.sk.serialize().as_ref(), &expected_sk[..]);
     keystore
 }