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/params.rs

@@ -18,7 +18,6 @@ use crate::traits::{AugmentClap, GetLogFilter};
 
 use std::path::PathBuf;
 use structopt::{StructOpt, clap::{arg_enum, App, AppSettings, _clap_count_exprs, SubCommand, Arg}};
-use client;
 
 pub use crate::execution_strategy::ExecutionStrategy;
 
@@ -44,6 +43,24 @@ impl Into<client::ExecutionStrategy> for ExecutionStrategy {
 	}
 }
 
+arg_enum! {
+	/// How to execute Wasm runtime code
+	#[allow(missing_docs)]
+	#[derive(Debug, Clone)]
+	pub enum WasmExecutionMethod {
+		// Uses an interpreter.
+		Interpreted,
+	}
+}
+
+impl Into<service::config::WasmExecutionMethod> for WasmExecutionMethod {
+	fn into(self) -> service::config::WasmExecutionMethod {
+		match self {
+			WasmExecutionMethod::Interpreted => service::config::WasmExecutionMethod::Interpreted,
+		}
+	}
+}
+
 arg_enum! {
 	/// Whether off-chain workers are enabled.
 	#[allow(missing_docs)]
@@ -404,6 +421,18 @@ pub struct RunCmd {
 	)]
 	pub offchain_worker: OffchainWorkerEnabled,
 
+	/// Method for executing Wasm runtime code.
+	#[structopt(
+		long = "wasm-execution",
+		value_name = "METHOD",
+		raw(
+			possible_values = "&WasmExecutionMethod::variants()",
+			case_insensitive = "true",
+			default_value = r#""Interpreted""#
+		)
+	)]
+	pub wasm_method: WasmExecutionMethod,
+
 	#[allow(missing_docs)]
 	#[structopt(flatten)]
 	pub execution_strategies: ExecutionStrategies,
@@ -651,6 +680,18 @@ pub struct ImportBlocksCmd {
 	#[structopt(flatten)]
 	pub shared_params: SharedParams,
 
+	/// Method for executing Wasm runtime code.
+	#[structopt(
+		long = "wasm-execution",
+		value_name = "METHOD",
+		raw(
+			possible_values = "&WasmExecutionMethod::variants()",
+			case_insensitive = "true",
+			default_value = r#""Interpreted""#
+		)
+	)]
+	pub wasm_method: WasmExecutionMethod,
+
 	/// The means of execution used when calling into the runtime while importing blocks.
 	#[structopt(
 		long = "execution",