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>

substrate/client/tests/common/in_instructions.rs

@@ -26,7 +26,9 @@ pub async fn provide_batch(batch: Batch) -> [u8; 32] {
   // TODO: Get the latest session
   let set = ValidatorSet { session: Session(0), network: batch.network };
   let pair = insecure_pair_from_name(&format!("ValidatorSet {:?}", set));
-  let keys = if let Some(keys) = serai.get_keys(set).await.unwrap() {
+  let keys = if let Some(keys) =
+    serai.get_keys(set, serai.get_latest_block_hash().await.unwrap()).await.unwrap()
+  {
     keys
   } else {
     let keys = (pair.public(), vec![].try_into().unwrap());