io: make io.cp use chunks (#9877)

vlib/io/io.v

@@ -1,15 +1,16 @@
 module io
 
 const (
-	buf_max_len = 5 * 1024
+	buf_max_len = 1024
 )
 
 pub fn cp(dst Writer, src Reader) ? {
-	mut buf := read_all(reader: src) or {
+	mut buf := []byte{len: buf_max_len}
+	for {
+		len := src.read(mut buf) or { break }
+		dst.write(buf[..len]) or {
 			return err
 		}
-	dst.write(buf) or {
-		return
 	}
 	unsafe {
 		buf.free()