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/service/test/src/lib.rs

@@ -178,6 +178,7 @@ fn node_config<G, E: Clone> (
 		chain_spec: (*spec).clone(),
 		custom: Default::default(),
 		name: format!("Node {}", index),
+		wasm_method: service::config::WasmExecutionMethod::Interpreted,
 		execution_strategies: Default::default(),
 		rpc_http: None,
 		rpc_ws: None,