Split Fixed uploads into chunks

Instead of reading the entire upload into a single string, we now return one chunk each time the underlying [read] call returns some data. This avoids using large amounts of RAM, hitting the 16 MB string limit on 32-bit systems. It also allows the application to begin processing the data before it has all arrived.

Fixes #153.

Possibly we should also adjust `Chunked` to stream the individual chunks rather than using the uploader's chunk size too, but this is enough to make it work with `curl -T`.
I'm pretty confused by this code.

`remaining` seems not to survive the call? But why is it written into then? Why is it a `ref` at all?

I don't understand the substance of the change other than replacing `read_exactly` with `read`. Apologies if it's obvious.