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/eth2_interop_keypairs/Cargo.toml

@@ -11,10 +11,10 @@ lazy_static = "1.4.0"
 num-bigint = "0.3.0"
 eth2_hashing = "0.1.0"
 hex = "0.4.2"
-milagro_bls = { git = "https://github.com/sigp/milagro_bls", tag = "v1.1.0" }
 serde_yaml = "0.8.11"
 serde = "1.0.110"
 serde_derive = "1.0.110"
+bls = { path = "../../crypto/bls" }
 
 [dev-dependencies]
 base64 = "0.12.1"