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/wasm/prepare.rs

@@ -19,10 +19,11 @@
 //! wasm module before execution. It also extracts some essential information
 //! from a module.
 
-use crate::wasm::env_def::ImportSatisfyCheck;
-use crate::wasm::PrefabWasmModule;
-use crate::{Schedule, Config};
-
+use crate::{
+	Schedule, Config,
+	chain_extension::ChainExtension,
+	wasm::{PrefabWasmModule, env_def::ImportSatisfyCheck},
+};
 use parity_wasm::elements::{self, Internal, External, MemoryType, Type, ValueType};
 use pwasm_utils;
 use sp_std::prelude::*;
@@ -355,6 +356,12 @@ impl<'a, T: Config> ContractModule<'a, T> {
 				return Err("module imports `seal_println` but debug features disabled");
 			}
 
+			if !T::ChainExtension::enabled() &&
+				import.field().as_bytes() == b"seal_call_chain_extension"
+			{
+				return Err("module uses chain extensions but chain extensions are disabled");
+			}
+
 			if import_fn_banlist.iter().any(|f| import.field().as_bytes() == *f)
 				|| !C::can_satisfy(import.field().as_bytes(), func_ty)
 			{