featurec-equivalent = should be commutative

[lvh]

#29 (implemented by #36) lets you express:

(= (:ip x) "10.0.0.1")

... but:

(= "10.0.0.1" (:ip x))

... should be equivalent. This is not quite as trivial as it may look at first glance, since one might also say:

(= (:ip x) (:ip y))

... but that's a different thing, and should compile down differently: the latter can't just be expressed as a single featurec, but instead is a fresh variable and two featurec relations (one for x, one for y).