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

consensus/state_processing/tests/tests.rs

@@ -1,11 +1,11 @@
-#![cfg(not(feature = "fake_crypto"))]
+// #![cfg(not(feature = "fake_crypto"))]
 
 use state_processing::{
     per_block_processing, test_utils::BlockBuilder, BlockProcessingError, BlockSignatureStrategy,
 };
 use types::{
-    AggregateSignature, BeaconState, ChainSpec, EthSpec, Keypair, MinimalEthSpec, Signature,
-    SignedBeaconBlock, Slot,
+    AggregateSignature, BeaconState, ChainSpec, EthSpec, Hash256, Keypair, MinimalEthSpec,
+    Signature, SignedBeaconBlock, Slot,
 };
 
 const VALIDATOR_COUNT: usize = 64;
@@ -92,15 +92,15 @@ where
 
 // TODO: use lazy static
 fn agg_sig() -> AggregateSignature {
-    let mut agg_sig = AggregateSignature::new();
-    agg_sig.add(&sig());
+    let mut agg_sig = AggregateSignature::infinity();
+    agg_sig.add_assign(&sig());
     agg_sig
 }
 
 // TODO: use lazy static
 fn sig() -> Signature {
     let keypair = Keypair::random();
-    Signature::new(&[42, 42], &keypair.sk)
+    keypair.sk.sign(Hash256::from_low_u64_be(42))
 }
 
 type TestEthSpec = MinimalEthSpec;