Some functions (e.g. find_frames) require considering multiple
compilation units, each of which might require their own split DWARF
lookup (and thus produce a continuation).
Operations that consult debug information may require additional files
to be loaded if split DWARF is being used. This enum returns the result
of the operation in the Output variant, or information about the split
DWARF that is required and a continuation to invoke once it is available
in the Load variant.
This trait represents a partially complete operation that can be resumed
once a load of needed split DWARF data is completed or abandoned by the
API consumer.