construct neutral fast interchange format

[flammie]

Omorfi starts to need pipelines to combine e.g. rule-based disambiguation with conll-u analysis, we can't have n-to-m lossy conversions on every tool. I think json might be good since many of the objects getting passed are basically lists of dicts of lists of dicts. But json needs to be read start to end and legacy formats process almost line-in-line-out.