srml: system: prune block hash mapping (#3062)

* srml: system: prune block number to hash mapping

* srml: system: add test for block hash mapping pruning

* node: bump runtime version

* srml: system: wrap long line

* srml: system: use parameter type for block hash count

* srml: system: prune block hash mapping before storage root calculation

* srml: system: keep the genesis hash in block number map

substrate/srml/balances/src/lib.rs

@@ -746,6 +746,7 @@ impl<T: Subtrait<I>, I: Instance> system::Trait for ElevatedTrait<T, I> {
 	type Lookup = T::Lookup;
 	type Header = T::Header;
 	type Event = ();
+	type BlockHashCount = T::BlockHashCount;
 }
 impl<T: Subtrait<I>, I: Instance> Trait<I> for ElevatedTrait<T, I> {
 	type Balance = T::Balance;

substrate/srml/balances/src/mock.rs

@@ -21,7 +21,7 @@
 use primitives::{traits::{IdentityLookup}, testing::Header};
 use substrate_primitives::{H256, Blake2Hasher};
 use runtime_io;
-use srml_support::{impl_outer_origin, traits::Get};
+use srml_support::{impl_outer_origin, parameter_types, traits::Get};
 use std::cell::RefCell;
 use crate::{GenesisConfig, Module, Trait};
 
@@ -65,6 +65,9 @@ impl Get<u64> for TransactionByteFee {
 // Workaround for https://github.com/rust-lang/rust/issues/26925 . Remove when sorted.
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct Runtime;
+parameter_types! {
+	pub const BlockHashCount: u64 = 250;
+}
 impl system::Trait for Runtime {
 	type Origin = Origin;
 	type Index = u64;
@@ -75,6 +78,7 @@ impl system::Trait for Runtime {
 	type Lookup = IdentityLookup<Self::AccountId>;
 	type Header = Header;
 	type Event = ();
+	type BlockHashCount = BlockHashCount;
 }
 impl Trait for Runtime {
 	type Balance = u64;