k3helpers: added utility function to convert vars to ids
k3new: filtermap now translates to filter if map has an id
typechecker: fix for flatten

A container of a container, when concatenated, should produce the inner type.
k3new: added transform to produce only folds and no ext/maps

This is due to a limitation in the k3new typechecker
There's also a paramter --folds_only to export_3new.py to use this.