Specs 1.0 roadmap

This is the tracking issue for the 1.0 release.

## Alpha phase

**Goal:** Get the API to a level that's supposedly stable and robust.

### Steps

* [ ] Check error handling for consistency
* [x] Make `World` an extension of `Resources` (#523) (worked on by @TimonPost)
* [ ] Evaluate `System::setup` (see #525, #437) (worked on by @LucioFranco)
* [x] How much should we keep? (everything ~~/ `Default` only / no automatic setup~~)
* [ ] Work out good practices
* [ ] Look into initialization story in general
* [ ] `RestrictedStorage` (worked on by @distransient)
* [x] Is it actually useful? How applicable?
* [ ] With which APIs does it intersect?
* [ ] Can its uage be improved?
* [x] Remove `common` module
* [ ] Remove experimental modules / move them out
* [ ] Allow joining over `FlaggedStorage` in parallel (worked on by @Moxinilian)
* [x] Audit uses of `unsafe` (https://github.com/slide-rs/specs/issues/548)
* [ ] Close, fix or postpone all existing issues
* [x] Try to cut down on dependencies where appropriate; document public dependencies and stabilize them before the alpha release
* [x] Migrate to Rust 2018

### Reflection

* [ ] Summarize all changes
* [ ] Release alpha version & create post

## Beta phase

**Goal:** Fix last inconsistencies / problems reported, improve docs quality, increase test coverage and create bigger demo.

### Fixes

* [ ] Plan eight weeks starting with the post for collecting problem reports.
* [ ] Fix bugs, fix problems / close issues which are out of scope; document what is in scope and what isn't.

### Documentation

* [ ] Improve top-level documentation; include only reference-style information and keep all explanations in the book; link to the book when appropriate.
* [ ] Ensure the following items are part of every item (where applicable)
* [ ] `# Errors` section
* [ ] `# Panics` section
* [ ] `# Examples` section

### Tests

* [ ] Set up codecov.io to measure code coverage
* [ ] Try to cover uncovered lines where appropriate
* [ ] Add larger simulation test

### Demo

* [ ] Create bigger demo with good documentation serving _the_ example, showing best practices, common patterns and general usage
* [ ] Make README example more useful / point to demo

### End of beta phase

* [ ] Release beta version and make another post
* [ ] Help a couple of projects upgrading

## 1.0

* [ ] If nothing goes horribly wrong, release `1.0` and make an announcement.