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_key_derivation/src/lib.rs

@@ -6,6 +6,6 @@ mod lamport_secret_key;
 mod plain_text;
 mod secret_bytes;
 
-pub use bls::SecretHash;
+pub use bls::ZeroizeHash;
 pub use derived_key::DerivedKey;
 pub use plain_text::PlainText;