Add fn properties test helpers

[dball]

This idea came to me in the shower earlier today, no idea how useful it might be in this form, but I thought I'd toss it out there for rumination.

I think a stable of fns to express common fn properties could be pretty useful.

[gfredericks]

I don't think this works because the :args key passed into the :fn function is conformed into a map, it's not a sequence.

[gfredericks]

which is problematic for commutative and associative; I'm not sure you can reconstruct the arguments from the conformed value

[dball]

s/unform oughta do it, right?

[gfredericks]

you'd need to have the :args spec accessible too then, right?