Should "io" in parsing instances support creating "ByteBufferKaitaiStream"?
From my understanding, `io`-attributes of parsing instances only support already available subclasses of `KaitaiStream`. Thinking about different implementations for my #546, I came up with a solution only having a plain `byte[]` available from which I would like to parse a type directly. Currently I'm using an empty type only to store data and providing `_io` of that type as the necessary subclass of `KaitaiStream` to some other parsing instance.

How about KS would generate creating an intermediate `ByteBufferKaitaiStream` automatically, so that one can parse from plain `byte[]` directly? Should be pretty much what KS already does for types in `seq`.

        this._raw_omsRecWoPreProc = this._io.readBytes(omsRecLength());
        KaitaiStream _io__raw_omsRecWoPreProc = new ByteBufferKaitaiStream(_raw_omsRecWoPreProc);
        this.omsRecWoPreProc = new FmtRecOmsWoPreProc(_io__raw_omsRecWoPreProc);

vs.

    KaitaiStream io = omsRecWoPreProc()._io();

Changing that single line to what is generated for `_read` already might be enough?

    seq:
      - id:   oms_rec_wo_pre_proc
        size: oms_rec_length
        type: fmt_rec_oms_wo_pre_proc
        
    instances:
      oms_rec:
        io:       oms_rec_wo_pre_proc._io
        pos:      0
        size:     oms_rec_length
        type:     fmt_rec_oms
        process:  de.am_soft.sm_mtg.format.parser.oms.pre_proc.fmt_opp_dispatcher

vs.
        
    seq:
      - id:       oms_rec_pre_procd
        size:     oms_rec_length
        process:  de.am_soft.sm_mtg.format.parser.oms.pre_proc.fmt_opp_dispatcher
        
    instances:
      oms_rec:
        io:       oms_rec_pre_procd
        type:     fmt_rec_oms