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_keypair.rs

@@ -0,0 +1,54 @@
+use crate::{
+    generic_public_key::{GenericPublicKey, TPublicKey},
+    generic_secret_key::{GenericSecretKey, TSecretKey},
+    generic_signature::TSignature,
+};
+use std::fmt;
+use std::marker::PhantomData;
+
+/// A simple wrapper around `PublicKey` and `GenericSecretKey`.
+#[derive(Clone)]
+pub struct GenericKeypair<Pub, Sec, Sig> {
+    pub pk: GenericPublicKey<Pub>,
+    pub sk: GenericSecretKey<Sig, Pub, Sec>,
+    _phantom: PhantomData<Sig>,
+}
+
+impl<Pub, Sec, Sig> GenericKeypair<Pub, Sec, Sig>
+where
+    Pub: TPublicKey,
+    Sec: TSecretKey<Sig, Pub>,
+    Sig: TSignature<Pub>,
+{
+    /// Instantiate `Self` from a public and secret key.
+    ///
+    /// This function does not check to ensure that `pk` is derived from `sk`. It would be a logic
+    /// error to supply such a `pk`.
+    pub fn from_components(pk: GenericPublicKey<Pub>, sk: GenericSecretKey<Sig, Pub, Sec>) -> Self {
+        Self {
+            pk,
+            sk,
+            _phantom: PhantomData,
+        }
+    }
+
+    /// Instantiates `Self` from a randomly generated secret key.
+    pub fn random() -> Self {
+        let sk = GenericSecretKey::random();
+        Self {
+            pk: sk.public_key(),
+            sk,
+            _phantom: PhantomData,
+        }
+    }
+}
+
+impl<Pub, Sec, Sig> fmt::Debug for GenericKeypair<Pub, Sec, Sig>
+where
+    Pub: TPublicKey,
+{
+    /// Defers to `self.pk` to avoid leaking the secret key.
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.pk.fmt(f)
+    }
+}