Unify a variable with a compound term

[rmanhaeve]

A variable cannot be unified with a term built through PySwip:

>>> from pyswip.easy import Functor, Variable
>>> Variable().unify(Functor("g", 1)(1))
TypeError: Cannot unify Variable(23) with value <pyswip.easy.Term object ...>
    due to the value unknown type <class 'pyswip.easy.Term'>

Functor.__call__ returns a Term, but Variable._fun has a branch for an
atom, a string, an integer, a boolean, a float and a list, and none for a
term. A foreign predicate therefore has no way to return a compound term,
only an atom or a list.

This puts the term into a term reference and unifies that, the way a list is
already handled. PL_unify is added to the imports. The regression test
registers a foreign predicate that returns g(1), and fails before the
change with the TypeError above.

tests/test_issues.py::TestIssues::test_issue_15 fails on my machine both
before and after, so it looks unrelated.

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