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/tokens/pallet/src/lib.rs

@@ -53,7 +53,7 @@ pub mod pallet {
     }
 
     pub fn mint(address: SeraiAddress, balance: Balance) {
-      // TODO: Prevent minting when it'd cause an amount exceeding the bond
+      // TODO: Prevent minting when it'd cause an amount exceeding the allocated stake
       AssetsPallet::<T>::mint(
         RawOrigin::Signed(ADDRESS.into()).into(),
         balance.coin,