Extract execution engines definitions into their own crates (#4489)

* Clean imports in wasmi_execution

* Replace `interpret_runtime_api_result` with `pointer_and_len_from_u64`.

* Extract sc-executor-common crate

* Extract `sc-executor-wasmi` into its own crate

* Extract `sc-executor-wasmtime` into its own crate.

* Add missing headers.

* Clean and docs

* Docs for sc-executor-wasmi

* Expand a comment about sandboxing

* Fix assert_matches

* Rename (un)pack_ptr_and_len and move them into util module

* Remove wasmtime errors in sc-executor-common

substrate/client/executor/common/src/wasm_runtime.rs

@@ -0,0 +1,39 @@
+// Copyright 2019 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Definitions for a wasm runtime.
+
+use crate::error::Error;
+use sp_core::traits::Externalities;
+use sp_wasm_interface::Function;
+
+/// A trait that defines an abstract wasm runtime.
+///
+/// This can be implemented by an execution engine.
+pub trait WasmRuntime {
+	/// Attempt to update the number of heap pages available during execution.
+	///
+	/// Returns false if the update cannot be applied. The function is guaranteed to return true if
+	/// the heap pages would not change from its current value.
+	fn update_heap_pages(&mut self, heap_pages: u64) -> bool;
+
+	/// Return the host functions that are registered for this Wasm runtime.
+	fn host_functions(&self) -> &[&'static dyn Function];
+
+	/// Call a method in the Substrate runtime by name. Returns the encoded result on success.
+	fn call(&mut self, ext: &mut dyn Externalities, method: &str, data: &[u8])
+		-> Result<Vec<u8>, Error>;
+}