Untangle key purpose/substance (API breaking)

In practice, if you're using the Key.{continuous, discrete, meta}
or their magic equivalents, your code should work as usual.

Orange actually distinguishes between two characteristics in its
variables (naming mine):

- purpose: meta/class/feature
- substance: continuous/discrete (called 'type' in Orange)

It appears that in the CSV format the two are conflated and that
Orange tries to deduce the substance when unknown (not sure about
this; in any case, we don't specify what types the meta/class
features are)
Support tab-formatted Orange output
stac-util insert: Support inserting into mid-text