loxi of_g reorganisation

Reviewer: @lane
CC: @vaterlaus

This pull request reorganizes a significant portion of the loxi base infrastructure. of_g and c-specific functionality from loxi_frontend has been moved to c_gen. In its place, loxi_ir has gained more functionality (back references, information about length+offsets of objects, being able to walk the inheritance tree, etc.), and a 'unified' ir model has been created. There is a very small new module loxi_globals that provides the entry pointer to the ir and to the list of versions.