Add lens/idempotence properties

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).