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/Cargo.toml

@@ -7,10 +7,11 @@ edition = "2018"
 [features]
 # `ef_tests` feature must be enabled to actually run the tests
 ef_tests = []
+milagro = ["bls/milagro"]
 fake_crypto = ["bls/fake_crypto"]
 
 [dependencies]
-bls = { path = "../../crypto/bls" }
+bls = { path = "../../crypto/bls", default-features = false }
 compare_fields = { path = "../../common/compare_fields" }
 ethereum-types = "0.9.1"
 hex = "0.4.2"