`atlas.um` and `tilemap.um` updates

Summary of changes: 1. Moved `tilemap::mk` to `tilemap::mk2`, `tilemap::mk` will accept width and height, then create the cells set to 0. 2. Added `Tilemap.get`. 3. `atlas.um` uses 1-based indexing, where 0 is an "air" tile, so it coincides with `tilemap.um`. This change is quite convenient, but I'd like to discuss about it, since I'm not too sure about it myself. 4. `atlas.um` no longer returns errors. Use `Atlas.has` or `Atlas.hasIndex` to check if an index exists. 5. Added `atlas::mk2` to make an atlas based on the cell size. 6. Added `Atlas.index` to get an index from a 2D coordinate. Caveats: If we go with the to the 1-based indexing change, should `hasIndex` return `true` on `0` or should it return `false`? Currently it returns `false`.