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/mod.rs

@@ -34,14 +34,13 @@ mod code_cache;
 mod prepare;
 mod runtime;
 
-use self::runtime::Runtime;
 use self::code_cache::load as load_code;
 use pallet_contracts_primitives::ExecResult;
 
 pub use self::code_cache::save as save_code;
 #[cfg(feature = "runtime-benchmarks")]
 pub use self::code_cache::save_raw as save_code_raw;
-pub use self::runtime::ReturnCode;
+pub use self::runtime::{ReturnCode, Runtime, RuntimeToken};
 
 /// A prepared wasm module ready for execution.
 #[derive(Clone, Encode, Decode)]