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

.github/workflows/test-suite.yml

@@ -77,7 +77,7 @@ jobs:
     - uses: actions/checkout@v1
     - name: Get latest version of stable Rust
       run: rustup update stable
-    - name: Run eth2.0-spec-tests with and without fake_crypto
+    - name: Run eth2.0-spec-tests with blst, milagro and fake_crypto
       run: make test-ef
   dockerfile-ubuntu:
     name: dockerfile-ubuntu