contracts: Add seal_rent_status (#8780)

* Move public functions up in rent.rs

* Added RentStatus

* Fix test name for consistency

Co-authored-by: Michael Müller <michi@parity.io>

* Mark rent functions as unstable

* Add unstable interfaces to README

* Fix doc typos

Co-authored-by: Andrew Jones <ascjones@gmail.com>

* Use DefaultNoBound

* Simplify calc_share(1)

* Don't output empty debug messages

* Make `seal_debug_message` unstable

Co-authored-by: Michael Müller <michi@parity.io>
Co-authored-by: Andrew Jones <ascjones@gmail.com>

substrate/frame/contracts/src/gas.rs

@@ -23,6 +23,7 @@ use frame_support::{
 		DispatchResultWithPostInfo, PostDispatchInfo, DispatchErrorWithPostInfo, DispatchError,
 	},
 	weights::Weight,
+	DefaultNoBound,
 };
 use sp_core::crypto::UncheckedFrom;
 
@@ -73,6 +74,7 @@ pub struct ErasedToken {
 	pub token: Box<dyn Any>,
 }
 
+#[derive(DefaultNoBound)]
 pub struct GasMeter<T: Config> {
 	gas_limit: Weight,
 	/// Amount of gas left from initial gas limit. Can reach zero.
@@ -82,18 +84,6 @@ pub struct GasMeter<T: Config> {
 	tokens: Vec<ErasedToken>,
 }
 
-impl<T: Config> Default for GasMeter<T> {
-	fn default() -> Self {
-		Self {
-			gas_limit: Default::default(),
-			gas_left: Default::default(),
-			_phantom: Default::default(),
-			#[cfg(test)]
-			tokens: Default::default(),
-		}
-	}
-}
-
 impl<T: Config> GasMeter<T>
 where
 	T::AccountId: UncheckedFrom<<T as frame_system::Config>::Hash> + AsRef<[u8]>