Intermediate commit, much more work to be done.

Just realized that the datastructures are not what's actually required,
making coding more cumbersome.
Setup the gather part to work with changed ChannelWriter.

Still missing is the unpacking of the data structure to generate
write error information.
It's easy, yet has to be done with care.
Implemented previously missins result handling.

However, it really shows I am trying to workaround the requirement
to prevent interleaved parallel writes, which in fact shouldn't be
a problem, as long as enough channelWriters can be pushed
Greatly simplified gather algorithm

After all, this special handling wasn't require, as our simple channels
can already handle that gracefully and efficiently.
First trivial implementation of a sequential parallel writer.

Next is syntax fixing
Syntax fixes
Now that it is implemented, I realize it can't work the way intended

We can't burst full files without caching them, it's that easy.
This is why we have to distribute buffers to multiple open files
one after another, and rely on the filesystem to deal with this.
After all, it wasn't slow that way either ...
Changed implementation back to unbuffered interleaved writes.

Kept some improvements to data structures though, after all there wasn't
much to be changed there
AbAbooMerge branch 'seq-writer'
Aborting failed experiment with sequential writer.

Some improvements have been made nonetheless.
Reduced heap allocations in case of writes.
Now the entire datastructure for writing is preset and pre-allocated.

Heap work is minimized that way