* Contain all layer types in an enum

* Implement `as_xlayer` for `LayerType`

* Make id optional

* Rename `ty` to `layer_type`

* Remove `layer_index`

* Move `as_x` functions to tests

* Fix build

* Expose types at the top level

While the modules are great for organizing the code internally, it is more
convenient for users of the create when the types are available at the top
level. This avoids the verbosity like "map::Map".

* Changelog updated

Removed the lines stating where which class has been moved, since all these
classes are not available at the top level again.

Also removed some things which I didn't think we actually noteworthy for people
reading up on what had changed.

* Restored some changelog items

Removal of these items unrelated to how the data types are exposed.

This simplifies code and should be better on memory usage since it is
less fragmented.

Also added `width` and `height` attributes to `Layer`.

Closes #54

Closes #116

Remove Map::source, add Tileset::source, change Image::source to PathBuf