The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
RepeatableReaderSized has a pre-allocated buffer which should help
with memory usage - before it grew the buffer. Since we know the size
of the chunks, pre-allocating it should be much more efficient.
RepeatableReaderBuffer uses the buffer passed in.
RepeatableLimit* are convenience funcitions for wrapping a reader in
an io.LimitReader and then a RepeatableReader with the same buffer
size.
