Reuse wasmtime instances, the PR (#5567)

* Preserve a single wasmtime instance.

* Sketch of wasm instance reusing.

* Refactor and docs.

* Rename state_snapshot to util module.

* Renaming.

* Comments.

* Error handling

* More fixes.

substrate/client/executor/common/Cargo.toml

@@ -12,6 +12,7 @@ documentation = "https://docs.rs/sc-executor-common/"
 [dependencies]
 log = "0.4.8"
 derive_more = "0.99.2"
+parity-wasm = "0.41.0"
 codec = { package = "parity-scale-codec", version = "1.3.0" }
 wasmi = "0.6.2"
 sp-core = { version = "2.0.0-alpha.5", path = "../../../primitives/core" }