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

crypto/bls/src/get_withdrawal_credentials.rs

@@ -0,0 +1,14 @@
+use crate::PublicKey;
+use eth2_hashing::hash;
+use ssz::Encode;
+
+/// Returns the withdrawal credentials for a given public key.
+///
+/// Used for submitting deposits to the Eth1 deposit contract.
+pub fn get_withdrawal_credentials(pubkey: &PublicKey, prefix_byte: u8) -> Vec<u8> {
+    let hashed = hash(&pubkey.as_ssz_bytes());
+    let mut prefixed = vec![prefix_byte];
+    prefixed.extend_from_slice(&hashed[1..]);
+
+    prefixed
+}