Map::layers() doesn't really list ALL layers - added a comment about it

[HughHoyland]

Took me a while to realize when I tried to load one especially large map.

I guess layers API needs a bit of improvement, but let's start with a comment.

[HughHoyland]

I see an even bigger problem.

While Layer<'a> is supposed to live as long as the Map, getting Layer out of a LayerType::Group(GroupLayer) only gives one a Layer<'a> where 'a is the lifetime of the local copy of GroupLayer.

Which means it cannot be passed outside of a function.

It is probably safe to cast... in an unsafe block.

[aleokdev]

I see an even bigger problem.

While Layer<'a> is supposed to live as long as the Map, getting Layer out of a LayerType::Group(GroupLayer) only gives one a Layer<'a> where 'a is the lifetime of the local copy of GroupLayer.

This is a bug. I see the problem: get_layer returns Layer<'self> instead of Layer<'map> + 'map. Has happened elsewhere before. Sorry about that, I'll submit a fix ASAP.

[aleokdev]

Looks good, thanks!
Please also add your changes to the CHANGELOG.md file under a new section:

## [Unreleased]
### Changed
- Improved documentation on `Map::layers` and `Map::get_layer`. (#306)

[HughHoyland]

Added the changelog.

[aleokdev]

Thank you!