Refactor NativeExecutor to support multiple Wasm execution methods (#3677)

* executor: Move definitions of externals out of wasm_executor module.

* executor: Create WasmRuntime trait.

This will be used to decouple the runtime cache from wasmi execution.

* executor: Remove WasmExecutor and move methods to wasmi_execution.

These will now be crate-internal functions and there is no need
for the struct.

* executor: Set default default_heap_pages in NativeExecutor.

* cli: CLI configuration for Wasm execution method.

* executor: Remove wasmi-specific code from wasm_runtime.

* Respond to review comments.

substrate/core/cli/src/lib.rs

@@ -380,6 +380,7 @@ impl<'a> ParseAndPrepareImport<'a> {
 		Exit: IntoExit
 	{
 		let mut config = create_config_with_db_path(spec_factory, &self.params.shared_params, self.version)?;
+		config.wasm_method = self.params.wasm_method.into();
 		config.execution_strategies = ExecutionStrategies {
 			importing: self.params.execution.into(),
 			other: self.params.execution.into(),
@@ -692,6 +693,8 @@ where
 			service::Roles::FULL
 		};
 
+	config.wasm_method = cli.wasm_method.into();
+
 	let exec = cli.execution_strategies;
 	let exec_all_or = |strat: params::ExecutionStrategy| exec.execution.unwrap_or(strat).into();
 	config.execution_strategies = ExecutionStrategies {