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/eth1/src/deposit_log.rs

@@ -64,7 +64,7 @@ impl DepositLog {
         };
 
         let signature_is_valid = deposit_pubkey_signature_message(&deposit_data, spec)
-            .map_or(false, |msg| deposit_signature_set(&msg).is_valid());
+            .map_or(false, |msg| deposit_signature_set(&msg).verify());
 
         Ok(DepositLog {
             deposit_data,