Specialize CombinationsWithReplacement::nth

[Philippe-Cholet]

Similarly to #914, I reorder CombinationsWithReplacement::next then specialize CombinationsWithReplacement::nth.

Benchmarks similar to #916 (comment)

cargo bench --bench specializations "combinations_with_replacement./(next|nth)"

Same allocations for:
1/next  [276.51 µs 277.04 µs 277.59 µs] [242.27 µs 242.93 µs 243.57 µs] [-13.937% -11.156% -8.0886%]
2/next  [278.45 µs 281.24 µs 284.58 µs] [251.73 µs 252.30 µs 252.90 µs] [-10.006% -9.0960% -8.1830%]
3/next  [275.74 µs 276.22 µs 276.80 µs] [227.55 µs 227.96 µs 228.36 µs] [-17.592% -16.847% -15.849%]
4/next  [265.08 µs 265.89 µs 266.76 µs] [241.05 µs 241.74 µs 242.47 µs] [-8.2684% -6.9087% -4.9878%]

Same allocations for:
1/nth/0 [2.7809 ms 2.7854 ms 2.7899 ms] [2.7524 ms 2.7612 ms 2.7709 ms] [-1.1968% -0.8696% -0.4725%]
2/nth/0 [2.7375 ms 2.7441 ms 2.7511 ms] [2.7761 ms 2.7871 ms 2.8002 ms] [+1.0923% +1.5657% +2.1318%]
3/nth/0 [2.8067 ms 2.8120 ms 2.8186 ms] [2.8385 ms 2.8534 ms 2.8709 ms] [+0.8478% +1.4726% +2.0929%]
4/nth/0 [2.5950 ms 2.5989 ms 2.6031 ms] [2.7731 ms 2.7777 ms 2.7830 ms] [+6.6335% +6.8806% +7.1368%]

-50% allocations for:
1/nth/1 [2.6836 ms 2.6882 ms 2.6929 ms] [1.4259 ms 1.4310 ms 1.4370 ms] [-46.934% -46.723% -46.471%]
2/nth/1 [2.7765 ms 2.7813 ms 2.7863 ms] [1.4193 ms 1.4270 ms 1.4355 ms] [-48.727% -48.488% -48.212%]
3/nth/1 [2.7947 ms 2.7994 ms 2.8045 ms] [1.4820 ms 1.4890 ms 1.4975 ms] [-46.859% -46.415% -45.916%]
4/nth/1 [2.6290 ms 2.6396 ms 2.6512 ms] [1.4378 ms 1.4436 ms 1.4532 ms] [-45.435% -45.103% -44.747%]

-67% allocations for:
1/nth/2 [2.7839 ms 2.7921 ms 2.8035 ms] [994.40 µs 999.99 µs 1.0072 ms] [-64.439% -64.203% -63.992%]
2/nth/2 [2.8508 ms 2.8547 ms 2.8589 ms] [986.24 µs 989.15 µs 992.24 µs] [-65.565% -65.432% -65.269%]
3/nth/2 [2.8938 ms 2.8980 ms 2.9023 ms] [1.0342 ms 1.0422 ms 1.0514 ms] [-64.243% -64.039% -63.847%]
4/nth/2 [2.7616 ms 2.7652 ms 2.7687 ms] [1.0504 ms 1.0536 ms 1.0568 ms] [-61.853% -61.580% -61.243%]

-80% allocations for:
1/nth/4 [2.7153 ms 2.7193 ms 2.7249 ms] [648.35 µs 650.19 µs 652.55 µs] [-75.952% -75.820% -75.684%]
2/nth/4 [2.7615 ms 2.7676 ms 2.7744 ms] [633.59 µs 634.88 µs 636.35 µs] [-76.885% -76.651% -76.382%]
3/nth/4 [2.8553 ms 2.8602 ms 2.8651 ms] [649.37 µs 652.42 µs 656.35 µs] [-77.134% -76.873% -76.551%]
4/nth/4 [2.7362 ms 2.7388 ms 2.7415 ms] [686.40 µs 688.99 µs 691.59 µs] [-74.836% -74.587% -74.255%]

-89% allocations for:
1/nth/8 [2.7403 ms 2.7499 ms 2.7600 ms] [431.48 µs 432.98 µs 434.67 µs] [-84.242% -84.024% -83.746%]
2/nth/8 [2.7270 ms 2.7574 ms 2.7969 ms] [398.89 µs 400.00 µs 401.43 µs] [-85.675% -85.462% -85.276%]
3/nth/8 [2.8713 ms 2.8850 ms 2.9016 ms] [417.89 µs 419.27 µs 420.81 µs] [-85.646% -85.553% -85.470%]
4/nth/8 [2.6630 ms 2.6712 ms 2.6812 ms] [460.50 µs 461.09 µs 461.68 µs] [-82.851% -82.770% -82.700%]

I did not expect next to be a bit faster but I'm not complaining.
.nth(0) is a little bit slower but other .nth(..)s are faster, closely following allocations changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.51%. Comparing base (6814180) to head (a250651).
Report is 59 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #923      +/-   ##
==========================================
+ Coverage   94.38%   94.51%   +0.12%     
==========================================
  Files          48       48              
  Lines        6665     6890     +225     
==========================================
+ Hits         6291     6512     +221     
- Misses        374      378       +4     

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[phimuemue]

Thanks @Philippe-Cholet. I think it's technically correct, although I'd rewrite some parts. If you like your proposal better, fine with me.

Should we extract the "initialization" part into an own function?