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_verify_msg.rs

@@ -4,6 +4,7 @@ use crate::cases::common::BlsCase;
 use bls::{PublicKey, Signature, SignatureBytes};
 use serde_derive::Deserialize;
 use std::convert::TryInto;
+use types::Hash256;
 
 #[derive(Debug, Clone, Deserialize)]
 pub struct BlsVerifyInput {
@@ -27,7 +28,9 @@ impl Case for BlsVerify {
 
         let signature_ok = (&self.input.signature)
             .try_into()
-            .map(|signature: Signature| signature.verify(&message, &self.input.pubkey))
+            .map(|signature: Signature| {
+                signature.verify(&self.input.pubkey, Hash256::from_slice(&message))
+            })
             .unwrap_or(false);
 
         compare_result::<bool, ()>(&Ok(signature_ok), &Some(self.output))