Add lens/idempotence properties

[ndmitchell]

Two useful properties of filepath operators are being idempotent (as you might hope normalise was) and being valid lenses, specifically obeying:

view l (set l b a)  = b
set l (view l a) a  = a
set l c (set l b a) = set l c a

I should probably add functions to capture these properties directly in the TestUtils code, then write things like lens getExtension replaceExtension for those which are actually true (I'm not sure extensions are, but some will be).