Combined locus/loci, info objects for genes & transcripts, reconstruct missing entries & columns for old releases

Ah, I see that it's used by the reconstruction code, but that code is still not very clear to me. What exactly are we doing when the `gene` feature is missing? We still have the `gene_id` attribute, so we're just grouping on that? Why/how do we have the `gene_id` attribute when we don't have the `gene` feature? (Clarifying all of that in comments would also be great.)