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.
This PR conflicts with David's #565 (he changed this bit of code) so I'll see how I can resolve the merge conflict in a way that makes sense. If possible, I'll adopt @vinniefalco's suggestion.