v1.18: implements weighted shuffle using N-ary tree (backport of #259)

[mergify]

This is port of firedancer's implementation of weighted shuffle:
https://github.com/firedancer-io/firedancer/blob/3401bfc26/src/ballet/wsample/fd_wsample.c

Problem

#185 implemented weighted shuffle using binary tree. Though asymptotically a
binary tree has better performance, compared to a Fenwick tree, it is
less cache local resulting in smaller improvements and in particular
slower WeightedShuffle::new.

Summary of Changes

In order to improve cache locality and reduce the overheads of
traversing the tree, this commit instead uses a generalized N-ary tree
with fanout of 16, showing significant improvements in both
WeightedShuffle::new and WeightedShuffle::shuffle.

With 4000 weights:

N-ary tree (fanout 16):

test bench_weighted_shuffle_new     ... bench:      36,244 ns/iter (+/- 243)
test bench_weighted_shuffle_shuffle ... bench:     149,082 ns/iter (+/- 1,474)

Binary tree:

test bench_weighted_shuffle_new     ... bench:      58,514 ns/iter (+/- 229)
test bench_weighted_shuffle_shuffle ... bench:     269,961 ns/iter (+/- 16,446)

Fenwick tree:

test bench_weighted_shuffle_new     ... bench:      39,413 ns/iter (+/- 179)
test bench_weighted_shuffle_shuffle ... bench:     364,771 ns/iter (+/- 2,078)

The improvements become even more significant as there are more items to
shuffle. With 20_000 weights:

N-ary tree (fanout 16):

test bench_weighted_shuffle_new     ... bench:     200,659 ns/iter (+/- 4,395)
test bench_weighted_shuffle_shuffle ... bench:     941,928 ns/iter (+/- 26,492)

Binary tree:

test bench_weighted_shuffle_new     ... bench:     881,114 ns/iter (+/- 12,343)
test bench_weighted_shuffle_shuffle ... bench:   1,822,257 ns/iter (+/- 12,772)

Fenwick tree:

test bench_weighted_shuffle_new     ... bench:     276,936 ns/iter (+/- 14,692)
test bench_weighted_shuffle_shuffle ... bench:   2,644,713 ns/iter (+/- 49,252)

---

This is an automatic backport of pull request #259 done by [Mergify](https://mergify.com).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.6%. Comparing base (e356a44) to head (05c205f).

Additional details and impacted files

@@            Coverage Diff            @@
##            v1.18     #429     +/-   ##
=========================================
- Coverage    81.6%    81.6%   -0.1%     
=========================================
  Files         827      827             
  Lines      224756   224753      -3     
=========================================
- Hits       183419   183404     -15     
- Misses      41337    41349     +12