Cleanups and surface reduction

Cleanup `STVector256`, `STPathSet`, `STPath` and `STPathElement`. Specifically:
 
* Don't use friendship unless needed
* Trim down interfaces
* Make classes feel more like std containers

This change is going to conflict with David's #565. I will resolve the merge conflicts once that is merged in, but would like early review anyways.

@scottschur, @tdfischer at your convenience please.
I don't believe mCompletePaths currently supports rbegin/rend.  It could of course be given that API.

Use of --i will change the logic here.  As written the loop will index mCompletePaths in reverse order.  With --i, the loop will skip i == 0.