Hide GIDs as internal details (#135)

* Partial commit

* Partial commit

* Partial commit

* Some suggested changes and fixed tests (except infinite)

* Replaced `tile` member in `LayerTileRef` with a function to get an
  `Option<&Tile>`.

* Replaced `Map::get_tile_by_gid` with `Map::get_tileset_for_gid`, which
  just returns the `Option<&Tileset>`. It also does a reverse search,
  which fixes the lookup in case an external tileset has grown since the
  map was last saved.

* Replaced `Tileset::get_tile_by_gid` with `Tileset::get_tile`, since
  subtracting of the `first_gid` already happens when creating the
  `LayerTileRef`. Also, we eventually should remove `first_gid` from
  `Tileset`, since it should be possible to share a single tileset
  instance betweeen several maps.

* Pre-allocate the tiles hash map for the expected number of tiles and
  use `or_default` instead of `or_insert_with(Default::default)`.

* [nonbuilding] Move ownership of tilesets
- Moves the ownership of tilesets from Map to an object implementing `TilesetCache`

* Clean up

* More cleanup

* Organize layers into modules

* Further modularization

* Add layer wrappers

* Implement `Clone + PartialEq + Debug` for wrappers

* Fix example

* Fix all tests except for test_infinite_tileset

* Move layer utils to its own module

* Better `Map::layers` documentation

* `TilesetCache` -> `cache::ResourceCache`

* Add `ResourcePath`, rename and add errors

* Interface changes
- Move embedded tilesets from cache to map
- Store `Option<LayerTileData>` instead of `LayerTileData`

* parser ->`&mut impl Iterator<Item=XmlEventResult>`

* Document that tilesets are ordered by first GID

* Fix the layer tiles using GIDs issue

* Run `cargo fix`

* Fix `test_infinite_tileset` tests

* Implement a way to access object tile data

* Rename `TiledWrapper` to `MapWrapper`

* More efficient `get_or_try_insert_tileset_with`

* Add `ResourcePathBuf`, use `Rc<Tileset>` in `Map`

* Remove `MapTileset::first_gid`

* Run `cargo fix`

* Remove unrelated `Tileset` changes

* Requested changes

* Avoid reference counting when accessing tiles

* Store tile data instead of (u32, u32)` in objects

* Remove unneeded functions from `ResourceCache`

* Address PR comments

* Misc improvements in `layers::tile`

* Improve example

* Convert `LayerTile` properties to fields

Co-authored-by: Thorbjørn Lindeijer <bjorn@lindeijer.nl>