feat: Store improvements (#88)

* feat: Store improvements

* Return `EmptyStore` error on empty store.
* Optimize failure on appending already known header.
* Remove custom genesis height. Now genesis is always height 1.
* Rename `*_unverified` methods to `*_unchecked`.
* Implement `append_single` and `append`.

* remove `StoreError::EmptyStore` and use `StoreError::NotFound` instead