Add "occurs-check" functionality

[rlouf]

I am trying to reproduce the examples in The Reasoned Schemer using kanren, and example 45 ("a variable cannot be equal to a list in which it occurs") hangs:

from kanren import var, run, eq

run(1, q, eq((q,), q))
# expected the empty tuple ()

The following also hangs:

from cons import cons
from kanren import var, run, eq

run(1, q, eq(cons(q, ()), q))
# expected an empty tuple

I am not sure this is a bug, so posting before investigating further.

[brandonwillard]

This looks like an "occurs-check"-related issue appearing in reify, because the unification state would be {q: (q,)} and reify would attempt to walk (q,) and replace the q inside it with another (q,), then walk that result, and so on.

[brandonwillard]

FYI: I labeled this a bug for the time being, but, since unification doesn't advertise an "occurs-check" feature/support, it's more of a feature request.