Delete struct snapshot and struct delta

Delete struct snapshot and struct delta

Weird code design.

I originally meant to privatize and de-heap these structures,
but it turns out they were not just used by a single module;
they were only used by `parse_metadata()`.

(Their domain seemed larger than it was, because they were being
initialized elsewhere, for no apparent reason.)

Then, while trying to clean up the global namespace, I noticed that the
hack was actually intertwined with another one: `parse_metadata()` was
being used for two purposes that are never actually used simultaneously.
This cluttered it.

So also separate `parse_metadata()` from `validate_metadata()`.