contracts: Allow contracts to dispatch calls into the runtime (#9276)

* contracts: Allow contracts to dispatch calls into the runtime

* Fix RPC tests

* Fix typo

* Replace () by AllowAllFilter and DenyAllFilter

* Add rust doc

* Fixup for `()` removal

* Fix lowest gas calculation

* Rename AllowAllFilter and DenyAllFilter

* Updated changelog

substrate/frame/support/src/traits/filter.rs

@@ -25,10 +25,20 @@ pub trait Filter<T> {
 	fn filter(_: &T) -> bool;
 }
 
-impl<T> Filter<T> for () {
+/// A [`Filter`] that allows any value.
+pub enum AllowAll {}
+
+/// A [`Filter`] that denies any value.
+pub enum DenyAll {}
+
+impl<T> Filter<T> for AllowAll {
 	fn filter(_: &T) -> bool { true }
 }
 
+impl<T> Filter<T> for DenyAll {
+	fn filter(_: &T) -> bool { false }
+}
+
 /// Trait to add a constraint onto the filter.
 pub trait FilterStack<T>: Filter<T> {
 	/// The type used to archive the stack.