Remove transformers, replace with custom layer on top of barback (#1592)

* Migrated all shared code out of the transformer files, and remove Transform arguments
* Removed transformers, replace with a layer on top of barback
* _buildAsset can synchronously return the map of assets that will be created
* use one scratchSpace for all builds, get shared libs outside of lib working
* make ScratchSpace use the common createFileFromStream method to limit number of open files
* update return type of getBuiltInTransformers back to Iterable<Set<Transformer>>