We need to reverse the iterator for getting the _youngest_ entry here.
Also seems to fix the issue that imag-diary edit -d <date> did not work
properly.
This patch rebuilds DiaryEntryIterator to be a wrapper around
StoreIdIterator and thus `Diary::entries()` to use `Store::entries` and
not `Store::retrieve_for_module()`.
The `Store::retrieve_for_module()` function is somehow buggy and loads
contents of the files into memory and _somehow_ causes the entries to be
rewritten without newlines.
This bug is fixed by moving away from `Store::retrieve_for_module()`.
This merge solved a _LOT_ of conflicts and was a rather complicated one,
as parts of the conflict-resolution involved rewriting of half the
stuff.
This merge commit fixes all the things so a `cargo check --all`
succeeds, but I did not yet check whether tests run without failure.
