fix: create TDFs larger than a single segment

[mkleene]

The previous API was logically incorrect and the only reason that we hadn't uncovered it was because we
only had payloads that had one segment.

Previously we'd write a manifest entry for each segment. Now we return a stream that clients can write into
for the payload. So the API changes to

var writer = new ZipWriter(out);
writer.data("entry one", new byte[] { ... });
try (var out = writer.stream("entry two")) {
  var fi = new FileInputStream("...");
  fi.transferTo(out);
}
writer.finish(); // write the zip manifest

Also change things so that we can handle streams that don't return all of the bytes requested during
a given read. Previously we assumed that a read would always return all of the bytes that might be available
in a stream.

Address #66