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/zeroize_hash.rs

@@ -0,0 +1,36 @@
+use super::SECRET_KEY_BYTES_LEN;
+use zeroize::Zeroize;
+
+/// Provides a wrapper around a `[u8; SECRET_KEY_BYTES_LEN]` that implements `Zeroize` on `Drop`.
+#[derive(Zeroize)]
+#[zeroize(drop)]
+pub struct ZeroizeHash([u8; SECRET_KEY_BYTES_LEN]);
+
+impl ZeroizeHash {
+    /// Instantiates `Self` with all zeros.
+    pub fn zero() -> Self {
+        Self([0; SECRET_KEY_BYTES_LEN])
+    }
+
+    /// Returns a reference to the underlying bytes.
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.0
+    }
+
+    /// Returns a mutable reference to the underlying bytes.
+    pub fn as_mut_bytes(&mut self) -> &mut [u8] {
+        &mut self.0
+    }
+}
+
+impl From<[u8; SECRET_KEY_BYTES_LEN]> for ZeroizeHash {
+    fn from(array: [u8; SECRET_KEY_BYTES_LEN]) -> Self {
+        Self(array)
+    }
+}
+
+impl AsRef<[u8]> for ZeroizeHash {
+    fn as_ref(&self) -> &[u8] {
+        &self.0
+    }
+}