executor: Migrate wasmtime backend to a high-level API (#4686)

* Migrate wasmtime backend to wasmtime-api

* Port to a newer version of wasmtime

* Update to the latest changes.

* Rejig the sandbox module a bit

* Materialze

* Fixes.

* executor wasm_runtime fix

* Refactor everything

* More refactoring

* Even more refactorings

* More cleaning.

* Update to the latest wasmtime

* Reformat

* Renames

* Refactoring and comments.

* Docs

* Rename FunctionExecutor to host.

* Imrpove docs.

* fmt

* Remove panic

* Assert the number of arguments are equal between wasmtime and hostfunc.

* Comment a possible panic if there is no corresponding value variant.

* Check signature of the entrypoint.

* Use git version of wasmtime

* Refine and doc the sandbox code.

* Comment RefCells.

* Update wasmtime to the latest-ish master.

This may solve a problem with segfaults.

* Apply suggestions from code review

Co-Authored-By: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* Use full SHA1 hash of wasmtime commit.

* Add a panic message.

* Add some documentation

* Update wasmtime version to include SIGSEGV fix

* Update to crates.io version of wasmtime

* Make it work.

* Move the creation of memory into `InstanceWrapper::new`

* Make `InstanceWrapper` !Send & !Sync

* Avoid using `take_mut`

* Update client/executor/wasmtime/Cargo.toml

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Limit maximum size of memory.

* Rename `init_state` to `with_initialized_state`

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/client/executor/wasmtime/src/lib.rs

@@ -16,10 +16,11 @@
 
 ///! Defines a `WasmRuntime` that uses the Wasmtime JIT to execute.
 
-mod function_executor;
+mod host;
 mod runtime;
-mod trampoline;
+mod state_holder;
+mod imports;
+mod instance_wrapper;
 mod util;
 
 pub use runtime::create_instance;
-