Function stitch_map

Working implementation. Unresolved issues:

- [ ] Check alignment of bounding boxes of neighboring tiles
- [ ] Tests (download maps with `with_mock`)
- [ ] Use empty (or random) maps in tests for better compression
- [ ] Align tiles at bottom-right of bounding box, to keep attribution visible
- [ ] Better file format for map files to support natural ordering
- [ ] Also save stitched file
- [ ] Parameter `crop = TRUE`

`stepwidth < 1` will be supported later.

Closes #2.