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/src/lib.rs

@@ -10,7 +10,6 @@
 mod builder;
 pub mod insecure_keys;
 mod manager;
-pub mod unencrypted_keys;
 mod validator_dir;
 
 pub use crate::validator_dir::{Error, Eth1DepositData, ValidatorDir, ETH1_DEPOSIT_TX_HASH_FILE};