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

Makefile

@@ -35,6 +35,7 @@ check-benches:
 run-ef-tests:
 	cargo test --release --manifest-path=$(EF_TESTS)/Cargo.toml --features "ef_tests"
 	cargo test --release --manifest-path=$(EF_TESTS)/Cargo.toml --features "ef_tests,fake_crypto"
+	cargo test --release --manifest-path=$(EF_TESTS)/Cargo.toml --features "ef_tests,milagro"
 
 # Runs only the tests/state_transition_vectors tests.
 run-state-transition-tests: