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`.
Yes, that makes sense. Thanks for clarification.

Although as a user, I wouldn't mind simply having a `length` lifted into the `chunk` type. With that, the result would contain all the information needed to correctly keep track of the remaining input length, and the API would not have to change.