Proposed change to naming convention for "dataful" `*G`s

Some of our *G typeclasses are different than others: they contain not just inG but actual relevant data; usually a gname but in the case of irisG also some further information about how the Iris program logic is being instantiated. These dataful classes come with a *PreG that represent their inG (dataless) part.

Dataful *Gs need to be treated differently, e.g. they have special initialization lemmas and they should not be bundled in library's *G as that leads to duplication of said data. So I propose to adjust our naming convention such that one can tell from the name whether a *G is dataful or not.

The new naming convention is up for bikeshedding; here are some proposals coming to my mind:

1. We call the dataful class *DataG and its inG-only part *G. So e.g. heapG → heapDataG and heapPreG → heapG.
2. We call the dataful class *DataG and its inG-only part *PreG. So e.g. heapG → heapDataG; heapPreG stays.
3. We call the dataful class *DG and its inG-only part *PreG. So e.g. heapG → heapDG; heapPreG stays.

I think I prefer (2) or (3) over (1) because it prevents confusion due to accidentally using the inG-only part, and also because it is easier for migration since we don't reuse an old name for a different purpose.

@robbertkrebbers @tchajed @jtassaro what do you think?