v1.18: implements weighted shuffle using binary tree (backport of #185)

[mergify]

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

Problem

Though Fenwick trees use less space, inverse queries require an
additional O(log n) factor for binary search resulting an overall
O(n log n log n) performance for weighted shuffle.

Summary of Changes

This commit instead uses a binary tree where each node contains the sum
of all weights in its left sub-tree. The weights themselves are
implicitly stored at the leaves. Inverse queries and updates to the tree
all can be done O(log n) resulting an overall O(n log n) weighted
shuffle implementation.

Based on benchmarks, this results in 24% improvement in
WeightedShuffle::shuffle:

Fenwick tree:

test bench_weighted_shuffle_new     ... bench:      36,686 ns/iter (+/- 191)
test bench_weighted_shuffle_shuffle ... bench:     342,625 ns/iter (+/- 4,067)

Binary tree:

test bench_weighted_shuffle_new     ... bench:      59,131 ns/iter (+/- 362)
test bench_weighted_shuffle_shuffle ... bench:     260,194 ns/iter (+/- 11,195)

Though WeightedShuffle::new is now slower, it generally can be cached
and reused as in Turbine:
https://github.com/anza-xyz/agave/blob/b3fd87fe8/turbine/src/cluster_nodes.rs#L68

Additionally the new code has better asymptotic performance. For
example with 20_000 weights WeightedShuffle::shuffle is 31% faster:

Fenwick tree:

test bench_weighted_shuffle_new     ... bench:     255,071 ns/iter (+/- 9,591)
test bench_weighted_shuffle_shuffle ... bench:   2,466,058 ns/iter (+/- 9,873)

Binary tree:

test bench_weighted_shuffle_new     ... bench:     830,727 ns/iter (+/- 10,210)
test bench_weighted_shuffle_shuffle ... bench:   1,696,160 ns/iter (+/- 75,271)<hr>This is an automatic backport of pull request #185 done by [Mergify](https://mergify.com).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.6%. Comparing base (cc4f37e) to head (204b10b).

Additional details and impacted files

@@           Coverage Diff           @@
##            v1.18     #425   +/-   ##
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  Coverage    81.5%    81.6%           
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  Files         827      827           
  Lines      224706   224756   +50     
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+ Hits       183329   183401   +72     
+ Misses      41377    41355   -22