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

common/validator_dir/src/builder.rs

@@ -170,7 +170,7 @@ impl<'a> Builder<'a> {
                     pubkey: voting_keypair.pk.clone().into(),
                     withdrawal_credentials,
                     amount,
-                    signature: Signature::empty_signature().into(),
+                    signature: Signature::empty().into(),
                 };
 
                 deposit_data.signature = deposit_data.create_signature(&voting_keypair.sk, &spec);
@@ -220,7 +220,7 @@ impl<'a> Builder<'a> {
                 // Write the withdrawal password to file.
                 write_password_to_file(
                     self.password_dir
-                        .join(withdrawal_keypair.pk.as_hex_string()),
+                        .join(withdrawal_keypair.pk.to_hex_string()),
                     withdrawal_password.as_bytes(),
                 )?;