misc ideas

[mlin]

• [x] unsafe branch-free versions of left & right
• [x] scan() micro-optimizations
• [ ] exclude outlier begs from domain width
• [x] eliminate outside_min_beg by switching outside_max_end to >= (benchmark)
• [x] work out theory for starting at internal nodes rather than leaves
• [ ] deal with outlier long intervals
• [ ] FBVector
• [ ] make regression loops vectorizable
• [ ] mmap & bsort
• [ ] factor out a little library for modeling rank CDF of any sorted array