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/wasmtime/src/lib.rs

@@ -0,0 +1,25 @@
+// 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/>.
+
+///! Defines a `WasmRuntime` that uses the Wasmtime JIT to execute.
+
+mod function_executor;
+mod runtime;
+mod trampoline;
+mod util;
+
+pub use runtime::create_instance;
+