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/src/lib.rs

@@ -31,19 +31,13 @@
 
 #[macro_use]
 mod wasm_utils;
-mod wasmi_execution;
 #[macro_use]
 mod native_executor;
-mod sandbox;
-mod allocator;
 pub mod deprecated_host_interface;
 mod wasm_runtime;
-#[cfg(feature = "wasmtime")]
-mod wasmtime;
 #[cfg(test)]
 mod integration_tests;
 
-pub mod error;
 pub use wasmi;
 pub use native_executor::{with_native_environment, NativeExecutor, NativeExecutionDispatch};
 pub use sp_version::{RuntimeVersion, NativeVersion};
@@ -54,6 +48,8 @@ pub use sp_core::traits::Externalities;
 pub use sp_wasm_interface;
 pub use wasm_runtime::WasmExecutionMethod;
 
+pub use sc_executor_common::{error, allocator, sandbox};
+
 /// Call the given `function` in the given wasm `code`.
 ///
 /// The signature of `function` needs to follow the default Substrate function signature.