archive: rename ParseSink to NARParseVisitor

- Rename the listener to not be called a "sink". If it were a "sink" it
  would be eating bytes and conform with any of the Nix sink stuff
  (maybe FileHandle should be a Sink itself! but that's a later CL's
  problem). This is a parser listener.
- Move the RetrieveRegularNARSink thing into store-api.cc, which is its
  only usage, and fix it to actually do what it is stated to do: crash
  if its invariants are violated.

  It's, of course, used to erm, unpack single-file NAR files, generated
  via a horrible contraption of sources and sinks that looks like a
  plumbing blueprint. Refactoring that is a future task.
- Add a description of the invariants of NARParseVisitor in preparation
  of refactoring it.

Change-Id: Ifca1d74d2947204a1f66349772e54dad0743e944

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