I just as little beyond the type as possible, so the implementation
changes this enables can be reviewed separately.

The downsides is that the coroutine has byte-by-byte loop transfer. Will
fix that next.

Rather than copying byte-by-byte, we let the coroutine know how much
data we would like it to send back to us.

We were calculating the nar hash wrong when the file ingestion method
was flat. I don't think there's anything we can do in that case but dump
the file again, so that's what I do.

As an optomization, we again could reuse the original dump for just the
recursive and non-sha256 case, but I rather do that after this fix, and
after my other PRs which deduplicate this code.

…from-dump-stream

This reverts commit cff2157.

I got it to just become `LocalStore::addToStoreFromDump`, cleanly taking
a store and then doing nothing too fancy with it.

`LocalStore::addToStore(...Path...)` is now just a simple wrapper with a
bare-bones sinkToSource of the right dump command.

This reverts commit 592851f. We don't
need this extra feature anymore

We use this to simplify `LocalStore::addToStoreFromDump`.

Also, hope I fixed build error with old clang (used in Darwin CI).

…systems/nix into from-dump-stream