use new pathTrail Iso lens

This patch allows diagrams-cairo to build against diagrams-lib HEAD.  I would push this to master myself, except I want to confirm the convention used.  

When we switch to lenses and stop exporting constructors, we prohibit the ugly `_pathTrails trs` expression, but also prohibit the pattern match `(Path trs)`.  @byorgey, is this what @jeffreyrosenbluth and I should do consistently?