ReferenceBlockSplittingVariantIterator

I would like to merge this so we don't have to use special branches in foghorn anymore.

This is a rebased version of #327 plus:
* an update to make the Variant mutators private until #320 is ready
* CMake updates

Thanks to @kgururaj for adding the N-base feature to fix #306
Why std::list here instead of std::vector? Since std::list is implemented as a doubly-linked list its performance characteristics are pretty awful compared to vector