Clean up splitstream reader implementation

Profiling cfsctl oci create-layer shows fully 5.4% of the run time of the program is spent in calloc() for the buffers for inline splitstream data, presumably zero-filling. Another 1% is spent in the corresponding free().

That's the result of this dumb code in read_splitstream_chunk():

            size => {
                let mut data = vec![0u8; size];
                reader.read_exact(&mut data)?;
                Ok(Some(SplitStreamData::Inline(data)))
            }

This looks like an obvious usecase for BorrowedCursor to avoid needing to initialize the memory, but the bigger win would come from not doing a new allocation each time: the caller never saves that buffer anyway, so we could use an internal Vec<u8> which we grow as-needed, returning a slice reference to it.

The main issue there is that read_splitstream_chunk() is a static method, so there's nowhere to store the buffer. We might consider merging this method into SplitStreamReader and figuring out an intelligent way to merge it with the existing inline_content: VecDeque<u8> that's already there.

containers / composefs-rs

Clean up splitstream reader implementation #9