S3 multipart is finally completely working. I still don't understand the
memory issue that stumped me yesterday, but rewrote the code to use a
simpler approach, which avoids the problem. Various other issues, and
testing it with large files, took all day.

This is now merged into the `s3-aws` branch, so when that branch lands, S3
support will massively improve, from the current situation of using a buggy
library that buffers uploaded files in memory, and cannot support very
large file uploads at all, to being able to support hopefully files of
arbitrary hugeness (at least up to a few terabytes).

BTW, thanks to Aristid Breitkreuz and Junji Hashimoto for working on the
multipart support in the `aws` library.
