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

testing/ef_tests/src/cases/bls_aggregate_sigs.rs

@@ -14,27 +14,27 @@ impl BlsCase for BlsAggregateSigs {}
 
 impl Case for BlsAggregateSigs {
     fn result(&self, _case_index: usize) -> Result<(), Error> {
-        let mut aggregate_signature = AggregateSignature::new();
+        let mut aggregate_signature = AggregateSignature::infinity();
 
         for key_str in &self.input {
             let sig = hex::decode(&key_str[2..])
                 .map_err(|e| Error::FailedToParseTest(format!("{:?}", e)))?;
-            let sig = Signature::from_bytes(&sig)
+            let sig = Signature::deserialize(&sig)
                 .map_err(|e| Error::FailedToParseTest(format!("{:?}", e)))?;
 
-            aggregate_signature.add(&sig);
+            aggregate_signature.add_assign(&sig);
         }
 
         // Check for YAML null value, indicating invalid input. This is a bit of a hack,
         // as our mutating `aggregate_signature.add` API doesn't play nicely with aggregating 0
         // inputs.
         let output_bytes = if self.output == "~" {
-            AggregateSignature::new().as_bytes().to_vec()
+            AggregateSignature::infinity().serialize().to_vec()
         } else {
             hex::decode(&self.output[2..])
                 .map_err(|e| Error::FailedToParseTest(format!("{:?}", e)))?
         };
-        let aggregate_signature = Ok(aggregate_signature.as_bytes().to_vec());
+        let aggregate_signature = Ok(aggregate_signature.serialize().to_vec());
 
         compare_result::<Vec<u8>, Vec<u8>>(&aggregate_signature, &Some(output_bytes))
     }