output/wavedrom: rephrase accumulation of output data

Iterate over the received sample set first, before iterating over the
respective sample's number of channels. This avoids redundant extraction
of sampled bits (which saves only little), but also increases locality
of processed data (though string accumulation still may be expensive).

It also adds the future option of RLE compression during accumulation of
output data, which perfectly matches the WaveDrom syntax for repeated
bit patterns.