Port to C++20.

[gatehouse]

This patch replaces various constructs that were deprecated in C++17 and removed in C++20 with their C++20 counterparts.

With this patch, s2geometry can be compiled with gcc 10.

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[jmr]

What's the minimum c++ version needed for this to work?

[gatehouse]

What's the minimum c++ version needed for this to work? @jmr I would expect this to work from C++14 upwards.

[jmr]

This isn't compiling:

https://travis-ci.org/github/google/s2geometry/builds/696839883

C++20 is too new to require. C++14 is definitely ok and C++17 is probably ok.

[jmr]

With this patch, s2geometry can be compiled with gcc 10.

gcc 10 works without this patch. https://travis-ci.org/github/google/s2geometry/jobs/722137049#L423

[jmr]

I will close and re-open this in hope of getting travis-ci to notice the .travis.yml changes. The current CMakeLists.txt is incompatible with the old .travis.yml.

[jmr]

This breaks both compilers that support c++20 https://travis-ci.org/github/google/s2geometry/jobs/722649190#L544

and those that don't https://travis-ci.org/github/google/s2geometry/jobs/722649187#L324