Consider replacing `utils._toposort` with the Python std. lib. `TopologicalSorter`

[majidaldo]

Consider replacing

unification/unification/utils.py

Line 20 in 94e489e

def _toposort(edges):

with graphlib.TopologicalSorter.

[brandonwillard]

We need to determine if TopologicalSorter could be worthwhile to adapt to this use-case.