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/src/beacon_chain.rs

@@ -862,7 +862,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                     root: target_root,
                 },
             },
-            signature: AggregateSignature::empty_signature(),
+            signature: AggregateSignature::empty(),
         })
     }
 
@@ -1654,7 +1654,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                 },
             },
             // The block is not signed here, that is the task of a validator client.
-            signature: Signature::empty_signature(),
+            signature: Signature::empty(),
         };
 
         per_block_processing(