Split classes into APIs and implementations

[haykam821]

Fabric API follows an API/implementation model, where each module is split into three packages:

• api package - Contains interfaces and utility classes that API consumers reference directly
• impl package - Contains implementations of the API that are references by helper methods in the API
• mixin package - Contains mixins necessary to make functionality work

Plasmid, and in particular its core game API, should more closely follow this API structure. Currently, some APIs use interfaces, but aren't placed in separate packages.