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.
See:
https://github.com/ripple/rippled/blob/develop/src/beast/beast/container/const_container.h#L29