Update package tools version + project structure

This release updates the project structure to make Sources, Tests, and Fixtures top level folders.
It also updates the package syntax to use Swift Package Manager tools version 4.0 which allows for the specification of source directories on a per-target basis. Now, when generating the xcodeproj it will no longer try to include the fixture files in your project. As this is a breaking change, the minor version has been increased. It also includes a bug fix for decoding Object schema, where objects were required to have properties (even if it was an empty dictionary). Now if the properties field is omitted, it's assumed to be an empty dictionary.