zstd: Resolve to a slice of slices to "execute"

[klauspost]

This could be an independent step between decode and execute or simply a part of decode.

Transform sequences into a slice of slices. The final step would then be:

func (s *sequenceDecs) executeSlices(seqs [][]byte) {
	s.out = s.out[:0]
	for _, seq := range seqs {
		s.out = append(s.out, seq...)
	}
}

The provided slices would simply be a mix of literal/backreference/dict/self-reference slices. This would be a serial process, but still simpler than the current execute.

The only check needed - beside sanity checks on output size - would be for self-references.