pq programs with arbitrary number of filters

[iximiuz]

At the moment, a valid pq program must always start from a decoder step, then have 0 or 1 mapping (transform step), and then 0 or 1 query step, potentially followed by a formatter. While I don't see how multiple query steps can be added, a more generic format like decode | map | map | ... | map | query | format would make the pq language much more expressive. Actually, the very first decode step would just become a syntactic sugar for map { .0: decode }.

The change would require some moderate refactoring, of course.