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

beacon_node/rest_api/src/helpers.rs

@@ -99,7 +99,7 @@ pub fn parse_pubkey_bytes(string: &str) -> Result<PublicKeyBytes, ApiError> {
     if string.starts_with(PREFIX) {
         let pubkey_bytes = hex::decode(string.trim_start_matches(PREFIX))
             .map_err(|e| ApiError::BadRequest(format!("Invalid hex string: {:?}", e)))?;
-        let pubkey = PublicKeyBytes::from_bytes(pubkey_bytes.as_slice()).map_err(|e| {
+        let pubkey = PublicKeyBytes::deserialize(pubkey_bytes.as_slice()).map_err(|e| {
             ApiError::BadRequest(format!("Unable to deserialize public key: {:?}.", e))
         })?;
         Ok(pubkey)