Investigate whether calendrical calculations can be sped up

[robertbastian]

trafficstars

Paper: https://arxiv.org/abs/2102.06959 Talk: https://www.youtube.com/watch?v=0s9F4QWAl-E

Interesting slide:

I think we're using the Reingold/Dershowitz algorithms, so looks like we can do a lot better.

[sffc]

We have put some time into improving the performance of the Reingold-Dershowitz algorithms, and they are substantially faster in our Rust implementation than in Lisp for instance, but I don't know how the benchmarks compare.

[Manishearth]

We've started integrating some of the algorithms via https://github.com/unicode-org/icu4x/pull/6870.

As previously discussed, we're focusing on the algorithm improvements in fixed to/from iso code (but not as much the other ones; decided on a case by case basis)