* 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

Start updating README.md

Refactor & document

Make layer ids optional and make tile layer name optional (like object layers)

Update CI link
Remove things from "Things missing" that were added
Update example to use `parse_file`

Parity with tile/object layers

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

Layer id defaults to 0 if missing, which is normally unused.
Tile layer name defaults to an empty String if missing, same as object layers.

Add Github Actions

Support object properties.

Added MIT license file

Closes #66

Fixes #72

There is only one image per Tile OR per Tileset. Added columns field.

Add layer id parsing

Add support for multiline string properties