Discard Executor (#1855)

closes #622 

Pros:
* simpler interface, just functions:
`create_runtime_from_artifact_bytes()` and `execute_artifact()`

Cons:
* extra overhead of constructing executor semantics each time

I could make it a combination of
* `create_runtime_config(params)` (such that we could clone the
constructed semantics)
* `create_runtime(blob, config)`
* `execute_artifact(blob, config, params)`

Not sure if it's worth it though.

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

polkadot/node/core/pvf/src/testing.rs

@@ -29,20 +29,20 @@ pub fn validate_candidate(
 	code: &[u8],
 	params: &[u8],
 ) -> Result<Vec<u8>, Box<dyn std::error::Error>> {
-	use polkadot_node_core_pvf_execute_worker::Executor;
+	use polkadot_node_core_pvf_execute_worker::execute_artifact;
 	use polkadot_node_core_pvf_prepare_worker::{prepare, prevalidate};
 
 	let code = sp_maybe_compressed_blob::decompress(code, 10 * 1024 * 1024)
 		.expect("Decompressing code failed");
 
 	let blob = prevalidate(&code)?;
-	let compiled_artifact_blob = prepare(blob, &ExecutorParams::default())?;
+	let executor_params = ExecutorParams::default();
+	let compiled_artifact_blob = prepare(blob, &executor_params)?;
 
-	let executor = Executor::new(ExecutorParams::default())?;
 	let result = unsafe {
 		// SAFETY: This is trivially safe since the artifact is obtained by calling `prepare`
 		//         and is written into a temporary directory in an unmodified state.
-		executor.execute(&compiled_artifact_blob, params)?
+		execute_artifact(&compiled_artifact_blob, &executor_params, params)?
 	};
 
 	Ok(result)