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

validator_client/slashing_protection/src/slashing_database.rs

@@ -148,7 +148,7 @@ impl SlashingDatabase {
             let mut stmt = txn.prepare("INSERT INTO validators (public_key) VALUES (?1)")?;
 
             for pubkey in public_keys {
-                stmt.execute(&[pubkey.as_hex_string()])?;
+                stmt.execute(&[pubkey.to_hex_string()])?;
             }
         }
         txn.commit()?;
@@ -163,7 +163,7 @@ impl SlashingDatabase {
     fn get_validator_id(txn: &Transaction, public_key: &PublicKey) -> Result<i64, NotSafe> {
         txn.query_row(
             "SELECT id FROM validators WHERE public_key = ?1",
-            params![&public_key.as_hex_string()],
+            params![&public_key.to_hex_string()],
             |row| row.get(0),
         )
         .optional()?