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/sr-api-macros/tests/runtime_calls.rs

@@ -186,7 +186,7 @@ fn record_proof_works() {
 
 	// Use the proof backend to execute `execute_block`.
 	let mut overlay = Default::default();
-	let executor = NativeExecutor::<LocalExecutor>::new(None);
+	let executor = NativeExecutor::<LocalExecutor>::new(WasmExecutionMethod::Interpreted, None);
 	execution_proof_check_on_trie_backend(
 		&backend,
 		&mut overlay,