unify printer and zone output

[hannesm]

at the moment, there are distinct zone output and a pretty printer, which results in pretty substantial amount of code, and also the output of log messages (using pp) is inconsistent with the common zone file format.

instead, unify them (maybe with a ?pretty flag to indicate whether to output e.g. algorithm names (for humans) or numbers (for interoperability)).

as a step afterwards, there should be test cases taking a zone file as input, and ensuing that it is correctly parsed, and also the textual output is the same as the input -- it may also be sensible to generate random zones, outputting them, decoding them, and checking equality.