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/generic_aggregate_public_key.rs

@@ -0,0 +1,18 @@
+use crate::{Error, PUBLIC_KEY_BYTES_LEN};
+
+/// Implemented on some struct from a BLS library so it may be used internally in this crate.
+pub trait TAggregatePublicKey: Sized + Clone {
+    /// Initialize `Self` to the infinity value which can then have other public keys aggregated
+    /// upon it.
+    fn infinity() -> Self;
+
+    /// Serialize `self` as compressed bytes.
+    fn serialize(&self) -> [u8; PUBLIC_KEY_BYTES_LEN];
+
+    /// Deserialize `self` from compressed bytes.
+    fn deserialize(bytes: &[u8]) -> Result<Self, Error>;
+}
+
+/*
+ * Note: there is no immediate need for a `GenericAggregatePublicKey` struct.
+ */