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/client/src/lib.rs

@@ -49,7 +49,7 @@
 //! use substrate_client::{Client, in_mem::Backend, LocalCallExecutor};
 //! use primitives::Blake2Hasher;
 //! use sr_primitives::{StorageOverlay, ChildrenStorageOverlay};
-//! use executor::NativeExecutor;
+//! use executor::{NativeExecutor, WasmExecutionMethod};
 //!
 //! // In this example, we're using the `Block` and `RuntimeApi` types from the
 //! // `substrate-test-runtime-client` crate. These types are automatically generated when
@@ -62,7 +62,7 @@
 //! 	backend.clone(),
 //! 	LocalCallExecutor::new(
 //! 		backend.clone(),
-//! 		NativeExecutor::<LocalExecutor>::new(None),
+//! 		NativeExecutor::<LocalExecutor>::new(WasmExecutionMethod::Interpreted, None),
 //! 		None,
 //!		),
 //! 	// This parameter provides the storage for the chain genesis.