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/eth2_libp2p/src/rpc/protocol.rs

@@ -33,13 +33,13 @@ lazy_static! {
     // same across different `EthSpec` implementations.
     pub static ref SIGNED_BEACON_BLOCK_MIN: usize = SignedBeaconBlock::<MainnetEthSpec> {
         message: BeaconBlock::empty(&MainnetEthSpec::default_spec()),
-        signature: Signature::empty_signature(),
+        signature: Signature::empty(),
     }
     .as_ssz_bytes()
     .len();
     pub static ref SIGNED_BEACON_BLOCK_MAX: usize = SignedBeaconBlock::<MainnetEthSpec> {
         message: BeaconBlock::full(&MainnetEthSpec::default_spec()),
-        signature: Signature::empty_signature(),
+        signature: Signature::empty(),
     }
     .as_ssz_bytes()
     .len();