Staking pallet (#373)

* initial staking pallet

* add staking pallet to runtime

* support session rotation for serai

* optimizations & cleaning

* fix deny

* add serai network to initial networks

* a few tweaks & comments

* fix some pr comments

* Rewrite validator-sets with logarithmic algorithms

Uses the fact the underlying DB is sorted to achieve sorting of potential
validators by stake.

Removes release of deallocated stake for now.

---------

Co-authored-by: Luke Parker <lukeparker5132@gmail.com>

tests/coordinator/src/tests/key_gen.rs

@@ -164,7 +164,11 @@ pub async fn key_gen<C: Ciphersuite>(
     }
   }
   assert_eq!(
-    serai.get_keys(set).await.unwrap().unwrap(),
+    serai
+      .get_keys(set, serai.get_block_by_number(last_serai_block).await.unwrap().unwrap().hash())
+      .await
+      .unwrap()
+      .unwrap(),
     (Public(substrate_key), network_key.try_into().unwrap())
   );