Adds support for :anonymous nodes

Added

• Better error description for invalid union types.

Changed

• Replaced auto-generated schema names like zero and one with :anonymous making it easier for a consuming project to decide what to do when a subschema does not have an explicit name. (#97)
• Replaced stringly-typed type properties for several schema types to instead use specific atoms corresponding to the set of valid JSON value type.

Fixed

• Bumped the elixir version and all of the project dependencies.