Extract simple features-like interface into a C++ only API

[paleolimbot]

This is a bit ambitious, but was inspired by Python folks interested in having some of these operations accessible from Python (see geopandas/community#10). The approach here is to use C++ rather than define a C API, although a C API could probably be wrapped around it if C linking were ever needed.

In addition to potentially benefiting the Python community, I'm hoping that having a wider community interested in helping to maintain the C++ that interfaces with S2 since I am admittedly a self-taught plumber in the world of compiled code.

I think I found a good way to do this piecemeal while keeping the tests passing (define a NewGeography() method and use it on a growing number of functions). I'm pleasantly surprised at how well this went for the accessors, which are the hardest because they often require access to the underlying data (and the new API obscures the underlying data and relies mostly on the Shape and Region interfaces).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.09875% with 128 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.93%. Comparing base (0c74988) to head (3d33c13).
⚠️ Report is 263 commits behind head on main.

Files with missing lines	Patch %	Lines
src/s2-geography/geography.cpp	58.33%	30 Missing ⚠️
src/s2-geography/accessors-geog.cpp	82.52%	18 Missing ⚠️
src/s2-geography/constructor.hpp	88.31%	18 Missing ⚠️
src/s2-geography/build.cpp	92.14%	15 Missing ⚠️
src/s2-constructors-formatters.cpp	94.83%	14 Missing ⚠️
src/s2-geography/accessors.cpp	91.24%	12 Missing ⚠️
src/s2-geography/coverings.cpp	77.77%	8 Missing ⚠️
src/s2-geography/linear-referencing.cpp	75.75%	8 Missing ⚠️
src/s2-transformers.cpp	95.71%	3 Missing ⚠️
src/s2-geography/distance.cpp	96.55%	1 Missing ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
- Coverage   94.75%   93.93%   -0.82%     
==========================================
  Files          42       49       +7     
  Lines        3220     3463     +243     
==========================================
+ Hits         3051     3253     +202     
- Misses        169      210      +41     

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[paleolimbot]

@edzer This is a pretty big change! The gist of it is that everything in the R package is now R-specific, and anything that didn't need R is now separated out (into something that will eventually live in its own repo). We had excellent test coverage before (and I added a bit of test coverage as I went and noticed that some cases weren't tested), but there's still a chance that some wires got crossed.

Would you prefer to check this before I merge into main or should we do that before the next release? Another big ish change that we might want to do before the next release is to update the underlying S2 version (a new version was released about two weeks ago).