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/genesis.rs

@@ -63,7 +63,7 @@ mod tests {
 	);
 
 	fn executor() -> executor::NativeExecutor<Executor> {
-		executor::NativeExecutor::new(None)
+		executor::NativeExecutor::new(executor::WasmExecutionMethod::Interpreted, None)
 	}
 
 	fn construct_block(