worker threads;

* create `main` and `worker` bins. registered them in `Cargo.toml` and
  `index.html`;
* set up woker context properly;
* implement needed methods for serialization and deserealization.

*TODO*: *fix* _"can't deserialize an worker message: DeserializeAnyNotSupported"_ error.

worker;

With the default `Bincode` codec we have got the
`DeserializeAnyNotSupported` error. The custom codec uses `serde_json`
to encoded and decode messages.