I would choose converters, if only because it has "convert" in the name. There are no hard rules, just use your judgement.
I have a perl script that needs a Convert::Bencode perl module.
Convert::Bencode is: Functions for converting to/from bencoded strings
Looking in the current tree, it could go in converters/ or testproc/ -
how would I choose? By the description, I'm leaning towards textproc/ .
Please CC me on the reply, I'm not subscribed to pkgsrc-users.