Merge amethyst#686

686: Meta/685/release 0 16 1 r=azriel91 a=azriel91

Prep for amethyst#685.

Co-authored-by: Azriel Hoh <azriel91@gmail.com>

Merge amethyst#683

683: Maintenance/682/ongoing code maintenance r=azriel91 a=azriel91

Fixes amethyst#682,

## Checklist

* **n/a** I've added tests for all code changes and additions (where applicable)
* **n/a** I've added a demonstration of the new feature to one or more examples
* **n/a** I've updated the book to reflect my changes
* **n/a** Usage of new public items is shown in the API docs

## API changes

Updated `rayon` to `1.3.0`, and `uuid` to `0.8.1`.

Co-authored-by: Azriel Hoh <azriel91@gmail.com>

Merge amethyst#683

683: Maintenance/682/ongoing code maintenance r=azriel91 a=azriel91

Fixes amethyst#682,

## Checklist

* **n/a** I've added tests for all code changes and additions (where applicable)
* **n/a** I've added a demonstration of the new feature to one or more examples
* **n/a** I've updated the book to reflect my changes
* **n/a** Usage of new public items is shown in the API docs

## API changes

Updated `rayon` to `1.3.0`, and `uuid` to `0.8.1`.

Co-authored-by: Azriel Hoh <azriel91@gmail.com>

Merge amethyst#641

641: Bump version to 0.15.1 r=torkleyy a=torkleyy



Co-authored-by: Thomas Schaller <torkleyy@gmail.com>

Merge amethyst#606

606: Prepare for 0.15.0 release r=azriel91 a=torkleyy

## Checklist

* [x] I've added tests for all code changes and additions (where applicable)
* [x] I've added a demonstration of the new feature to one or more examples
* [x] I've updated the book to reflect my changes
* [x] Usage of new public items is shown in the API docs

## API changes

* Remove multiple methods from `WorldExt` which were moved to `shred::World` (see amethyst/shred#141)


Co-authored-by: Thomas Schaller <torkleyy@gmail.com>
Co-authored-by: Azriel Hoh <azriel91@gmail.com>

Update Changelog, bump versions

Merge amethyst#515

515: Allow accessing fetched entities of a `Storage` r=Xaeroxe a=torkleyy

I've added `Storage::entities` to allow making extension methods for `Storage` that return `Entity`.

This method is hidden because I'm 100% certain newcomers will be confused by this otherwise. This makes it less visible, yes, but I think that's worth for avoiding this confusion.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/slide-rs/specs/515)
<!-- Reviewable:end -->


Co-authored-by: Thomas Schaller <torkleyy@gmail.com>

Merge amethyst#457

457: Reduced the complexity of the entity allocator r=torkleyy a=Flecheck

This pull request uses a vector to store deleted entities to reduce the complexity of entity allocation to O(1)*.
The maximum memory footprint is 2x the maximum current amount but it can only happen in the worst case and is I believe acceptable for such a use case.

The current approach has the pitfall of often reusing the same entities, hence potentially making their generation quite high. Introducing a VecDequeue instead of the current Vec for the EntityCache could potentially solve this problem (with a cost in memory), so if you believe the change would be worth it I can add it relatively easily. However, this might be unnecessary as 2³¹-1 is the maximum generation.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/slide-rs/specs/457)
<!-- Reviewable:end -->


Co-authored-by: Flecheck <bessou.mathieu@gmail.com>

Merge amethyst#278

278: small adjustments in the documentation r=torkleyy a=hartmut

minor changes in the specs book