new-feature: add mathext/util: with max, min, clap utils

new-feature: add bitstr to store a bits serias whose size is non-8-al…

…igned.

new-feature: bitmap: add Builder.Set() to update one bit in a bitmap

fix: typo of pakcage name

new-feature: mathext.zipf: move zipf into dir zipf/ add Accesses() to…

… generate sequence of array access in zipf distribution

new-feature: mathext.zipf: move zipf into dir zipf/ add Accesses() to…

… generate sequence of array access in zipf distribution

new-feature: mathext: add Zipf to convert evenly distributed value to…

… zipf distributed

new-feautre: bitmap: add Select32R64(): select() with a rank64 index …

…for speeding up. it costs 7.8ns/op

new-feautre: bitmap: add Select32R64(): select() with a rank64 index …

…for speeding up. it costs 7.8ns/op

refactor: bitmap: use 256x8 table for select32. remove packed lookup …

…table [256]int32