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();

beacon_node/eth2_libp2p/tests/rpc_tests.rs

@@ -127,7 +127,7 @@ async fn test_blocks_by_range_chunked_rpc() {
     let empty_block = BeaconBlock::empty(&spec);
     let empty_signed = SignedBeaconBlock {
         message: empty_block,
-        signature: Signature::empty_signature(),
+        signature: Signature::empty(),
     };
     let rpc_response = Response::BlocksByRange(Some(Box::new(empty_signed)));
 
@@ -238,7 +238,7 @@ async fn test_blocks_by_range_chunked_rpc_terminates_correctly() {
     let empty_block = BeaconBlock::empty(&spec);
     let empty_signed = SignedBeaconBlock {
         message: empty_block,
-        signature: Signature::empty_signature(),
+        signature: Signature::empty(),
     };
     let rpc_response = Response::BlocksByRange(Some(Box::new(empty_signed)));
 
@@ -365,7 +365,7 @@ async fn test_blocks_by_range_single_empty_rpc() {
     let empty_block = BeaconBlock::empty(&spec);
     let empty_signed = SignedBeaconBlock {
         message: empty_block,
-        signature: Signature::empty_signature(),
+        signature: Signature::empty(),
     };
     let rpc_response = Response::BlocksByRange(Some(Box::new(empty_signed)));
 
@@ -479,7 +479,7 @@ async fn test_blocks_by_root_chunked_rpc() {
     let full_block = BeaconBlock::full(&spec);
     let signed_full_block = SignedBeaconBlock {
         message: full_block,
-        signature: Signature::empty_signature(),
+        signature: Signature::empty(),
     };
     let rpc_response = Response::BlocksByRoot(Some(Box::new(signed_full_block)));
 
@@ -598,7 +598,7 @@ async fn test_blocks_by_root_chunked_rpc_terminates_correctly() {
     let full_block = BeaconBlock::full(&spec);
     let signed_full_block = SignedBeaconBlock {
         message: full_block,
-        signature: Signature::empty_signature(),
+        signature: Signature::empty(),
     };
     let rpc_response = Response::BlocksByRoot(Some(Box::new(signed_full_block)));