contracts: Allow runtime authors to define a chain extension (#7548)

* Make host functions return TrapReason

This avoids the need to manually store any trap reasons
to the `Runtime` from the host function. This adds the following
benefits:

* It properly composes with the upcoming chain extensions
* Missing to set a trap value is now a compile error

* Add chain extension

The chain extension is a way for the contract author to add new
host functions for contracts to call.

* Add tests for chain extensions

* Fix regression in set_rent.wat fixture

Not all offsets where properly updated when changing the fixtures
for the new salt on instantiate.

* Pre-charge a weight amount based off the specified length

* Improve fn write docs

* Renamed state to phantom

* Fix typo

substrate/frame/contracts/src/lib.rs

@@ -89,6 +89,8 @@ mod wasm;
 mod rent;
 mod benchmarking;
 mod schedule;
+
+pub mod chain_extension;
 pub mod weights;
 
 #[cfg(test)]
@@ -320,6 +322,9 @@ pub trait Config: frame_system::Config {
 	/// Describes the weights of the dispatchables of this module and is also used to
 	/// construct a default cost schedule.
 	type WeightInfo: WeightInfo;
+
+	/// Type that allows the runtime authors to add new host functions for a contract to call.
+	type ChainExtension: chain_extension::ChainExtension;
 }
 
 decl_error! {
@@ -387,6 +392,10 @@ decl_error! {
 		TooManyTopics,
 		/// The topics passed to `seal_deposit_events` contains at least one duplicate.
 		DuplicateTopics,
+		/// The chain does not provide a chain extension. Calling the chain extension results
+		/// in this error. Note that this usually  shouldn't happen as deploying such contracts
+		/// is rejected.
+		NoChainExtension,
 	}
 }