Rename `canvas` to `contracts` (#1265)

* Update SVG

* Rename `canvas-kusama` folder `contracts-rococo`

* Search-Replace `canvas-kusama` with `contracts-rococo`

* Search-Replace `canvas_kusama` with `contracts_rococo`

* Search-Replace `canvas_rococo` with `contracts_rococo`

* Rename `canvas-rococo.json` to `contracts-rococo.json`

* Rename `CanvasKusamaChainSpec` to `ContractsRococoChainSpec`

* Migrate chain specs and configurations to `contracts-rococo`

* Adapt readme

* Improve directory structure

* Remove last occurrences of `canvas`

* Apply `cargo fmt`

* Update README.md

Co-authored-by: Alexander Theißen <[email protected]>

* Fix funny indentation

* Fix wasm path renaming

Co-authored-by: Alexander Theißen <[email protected]>
Co-authored-by: Wilfried Kopp <[email protected]>
Co-authored-by: Chevdor <[email protected]>