Further read optimization

[oliverpool]

Hi, first of all thank you for sharing your code!

I was quite impressed by the reading speed, and I think that I found a way to make this part a bit faster:

1brc/main.go

Lines 131 to 155 in 8513d5e

	// spawn a goroutine to read file in chunks and send it to the chunk channel for further processing
	go func() {
	buf := make([]byte, chunkSize)
	leftover := make([]byte, 0, chunkSize)
	for {
	readTotal, err := file.Read(buf)
	if err != nil {
	if errors.Is(err, io.EOF) {
	break
	}
	panic(err)
	}
	buf = buf[:readTotal]
	toSend := make([]byte, readTotal)
	copy(toSend, buf)
	lastNewLineIndex := bytes.LastIndex(buf, []byte{'\n'})
	toSend = append(leftover, buf[:lastNewLineIndex+1]...)
	leftover = make([]byte, len(buf[lastNewLineIndex+1:]))
	copy(leftover, buf[lastNewLineIndex+1:])
	chunkStream <- toSend

Currently each loop calls 2 times make([]byte) and copy. This could be reduced to 1, by storing the leftover as a length, instead of a byte slice:

		buf := make([]byte, chunkSize)
		leftover := 0
		for {
			n, err := file.Read(buf[leftover:]) // append to the leftover
			if err != nil {
				if errors.Is(err, io.EOF) {
					break
				}
				panic(err)
			}
			toSend := buf[:leftover+n]

			lastNewLineIndex := bytes.LastIndexByte(toSend, '\n')

			buf = make([]byte, chunkSize) // prepare a new buffer for next read
			leftover = copy(buf, toSend[lastNewLineIndex+1:])

			chunkStream <- toSend[:lastNewLineIndex+1]
		}

On a sample file, this code is about 10% faster than the current version