Add s2_cell_union() vector class

[paleolimbot]

This PR adds lower-level access to the S2CellUnion C++ class, which powers s2 indexing. It completes #85 and adds the functionality requested in #94.

Under the hood these are list(s2_cell()) (no external pointers).

library(s2)

geog <- s2_data_countries("Germany")
(covering <- s2_covering_cell_ids(geog))
#> <s2_cell_union[1]>
#> List of 1
#> $ : s2_cell[1:8] 4707, 470c, 4775, 4777, 4794, 479c, 47b , 47c4
interior_covering <- s2_covering_cell_ids(geog, interior = TRUE)

s2plot::s2plot(geog)
s2plot::s2plot(covering, add = T)
s2plot::s2plot(interior_covering, add = T)

as_s2_geography(covering)
#> <s2_geography[1]>
#> [1] <POLYGON ((6.0275303 49.8623037, 5.50935844 47.3168538, 8.47114463 47.1357861, 11.5441739 46.8652403, 13.1147522 46.6939166...>

Still TODO:

• Add some optimized methods since in theory working with cells is way faster than working with geographies

[codecov-commenter]

Codecov Report

Merging #164 (1f8564b) into main (a323a74) will increase coverage by 0.33%.
The diff coverage is 98.23%.

@@            Coverage Diff             @@
##             main     #164      +/-   ##
==========================================
+ Coverage   94.56%   94.89%   +0.33%     
==========================================
  Files          40       42       +2     
  Lines        2871     3176     +305     
==========================================
+ Hits         2715     3014     +299     
- Misses        156      162       +6     

Impacted Files	Coverage Δ
src/s2-cell.cpp	98.98% <93.33%> (-0.74%)	⬇️
R/s2-cell-union.R	97.91% <97.91%> (ø)
src/s2-cell-union.cpp	99.45% <99.45%> (ø)
R/s2-cell.R	100.00% <100.00%> (ø)
src/s2-transformers.cpp	96.56% <0.00%> (-0.01%)	⬇️
R/s2-transformers.R	100.00% <0.00%> (ø)

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