Read the value of both arguments of =/2 separately

[rmanhaeve]

A query that returns a term such as 1 = 1 raises an AttributeError:

>>> from pyswip import Prolog
>>> list(Prolog.query("X = [(1 = 1)]"))
AttributeError: 'int' object has no attribute 'value'

_unifier, the handler for =/2, reads args[0].value and args[1].value,
and falls back to the argument itself when it has no value. The fallback reads
args[0].value again, so it only recovers when the second argument has no
value. Any integer, float or string on the left of = raises in the fallback
as well.

This reads each argument on its own. Added a regression test, which fails
before the change and passes after it.

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

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