fragmenting_writer.go
func newWritableChunk(checksum Checksum, contents *typed.WriteBuffer) *writableChunk {
	return &writableChunk{
		size:     0,
		sizeRef:  contents.DeferUint16(),
		checksum: checksum,
		contents: contents,
	}
}
func newWritableChunk(checksum Checksum, contents *typed.WriteBuffer) *writableChunk {
	return &writableChunk{
		size:     0,
		sizeRef:  contents.DeferUint16(),
		checksum: checksum,
		contents: contents,
	}
}
// newWritableChunk creates a new writable chunk around a checksum and a buffer to hold data