Introduce toy runtime for testing inside substrate. (#66)

* Introduce simple blockchain runtime for substrate tests.

* Remove bad files.

* Add needed wasm binaries.

* Refactoring.

- Repot files in test-runtime.
- Rename troublesome `Joiner::join` to `Joiner::and`.
- Rework `Slicable` to dedup code.

* More fixes and refactoring

* Rebuild substrate test wasm.

* Fix merge errors.

* Rename the disasterously named `to_vec` to `encode`.

Also rename `as_slice_then` to `with_encoded`.

* Tests for toy runtime.

* Fix doc nit