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/src/per_block_processing/verify_deposit.rs

@@ -21,7 +21,7 @@ pub fn verify_deposit_signature(deposit_data: &DepositData, spec: &ChainSpec) ->
         .ok_or_else(|| error(DepositInvalid::BadBlsBytes))?;
 
     verify!(
-        deposit_signature_set(&deposit_signature_message).is_valid(),
+        deposit_signature_set(&deposit_signature_message).verify(),
         DepositInvalid::BadSignature
     );