Added a proper, modular implementation of LLX and SCX (common/scx_provider.h). It applies the weak descriptor technique described in my DISC 2017 paper. I used this implementation of LLX/SCX to simplify brown_ext_abtree_lf quite a bit. (The original implementation of this data structure, which has range query support, appears in ds/brown_ext_abtree_rq_lf.)