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

common/validator_dir/Cargo.toml

@@ -5,7 +5,6 @@ authors = ["Paul Hauner <paul@paulhauner.com>"]
 edition = "2018"
 
 [features]
-unencrypted_keys = []
 insecure_keys = []
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html