Reduce emissions by 10% every year (#108)

* deprecate top balance

* stub dynamic emissions impl

* impl 90% curve

* scaffold emissions rate function

* emissions curve

* implement new supply curve

* remove comments

* update comment

* bump version

* test reset

* add tests for emissions reduction

* test

* fix test

* bump version

api/src/state/bus.rs

@@ -18,6 +18,7 @@ pub struct Bus {
     pub theoretical_rewards: u64,
 
     /// The largest known stake balance seen by the bus this epoch.
+    #[deprecated(since = "2.8.0", note = "Top balance is no longer tracked or used")]
     pub top_balance: u64,
 }