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/snapshot_cache.rs

@@ -99,7 +99,7 @@ mod test {
     use super::*;
     use types::{
         test_utils::{generate_deterministic_keypair, TestingBeaconStateBuilder},
-        BeaconBlock, Epoch, MainnetEthSpec, Signature, SignedBeaconBlock, Slot,
+        BeaconBlock, Epoch, MainnetEthSpec, SignedBeaconBlock, Slot,
     };
 
     const CACHE_SIZE: usize = 4;
@@ -115,7 +115,9 @@ mod test {
             beacon_state_root: Hash256::from_low_u64_be(i),
             beacon_block: SignedBeaconBlock {
                 message: BeaconBlock::empty(&spec),
-                signature: Signature::new(&[42], &generate_deterministic_keypair(0).sk),
+                signature: generate_deterministic_keypair(0)
+                    .sk
+                    .sign(Hash256::from_low_u64_be(42)),
             },
             beacon_block_root: Hash256::from_low_u64_be(i),
         }