new directory structure

This is a proposal and new step towards a new project directory structure.

## Motivation

Our current directory structure is a Dart pub project structure mixed with things that have nothing to do with it, such as contrib, pub-server.go, boilerplate (license, readme, etc). It also does not support feature incubation. All code goes into the core directories and it is hard to tell unstable parts from the stable parts. We are also planning to develop sample apps showcasing Streamy features.

## Proposal

```
.
├ streamy/ - contains core streamy libraries (in pure pub format)
├ contrib/ - contains useful utilities that help working with the project
├ labs/ - where new major features are incubated
├ examples/ - sample apps showing how to use Streamy
├ LICENSE, README.md, VERSION
```

We should be mindful about what we put into the root folder.