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/beacon_chain/src/validator_pubkey_cache.rs

@@ -140,6 +140,7 @@ struct ValidatorPubkeyCacheFile(File);
 enum Error {
     Io(io::Error),
     Ssz(DecodeError),
+    PubkeyDecode(bls::Error),
     /// The file read from disk does not have a contiguous list of validator public keys. The file
     /// has become corrupted.
     InconsistentIndex {
@@ -200,7 +201,7 @@ impl ValidatorPubkeyCacheFile {
             let expected = last.map(|n| n + 1);
             if expected.map_or(true, |expected| index == expected) {
                 last = Some(index);
-                pubkeys.push((&pubkey).try_into().map_err(Error::Ssz)?);
+                pubkeys.push((&pubkey).try_into().map_err(Error::PubkeyDecode)?);
                 indices.insert(pubkey, index);
             } else {
                 return Err(Error::InconsistentIndex {