Add `Reaction.` namespace and deprecate ReactionCore

To avoid overpopulating of random Reaction camel cased exports we should adopt a cleaner convention using `Reaction.`   

Add exportable `Reaction` and move to a convention of:

```
Reaction.Core.
Reaction.Collections.
Reaction.Subscriptions
Reaction.Checkout
Reaction.Import
```

etc...

Existing namespace of  `ReactionCore`,`ReactionImport`, etc  can be wrapped, mapped with a warning added.
