Set the classes attribute for sfc_GEOMETRY

[paleolimbot]

The change here is still not quite right yet...it's supposed to be the same length as the vector, not just the unique classes of the geometries.

May fix r-spatial/sf#2234 if done correctly!

[paleolimbot]

@edzer Is there somewhere that attr(some_sfc, "classes") is documented? The issue is that wk is generating it improperly (which I should fix). If I understand it correctly, it should be a character vector with the same length as some_sfc as a cache of class(some_sfc[[i]]) for i in seq_along(some_sfc)?

[edzer]

Not documented (good point!) - sfc_GEOMETRY objects need to have it, and it contains the middle (second) value of the class of an sfg object:

> st_sfc(st_point(0:1), st_linestring(matrix(1:4,2))) |> attr("classes")
[1] "POINT"      "LINESTRING"

AFAICS it's only used when converting to WKB.

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (e244165) 99.02% compared to head (f70b650) 99.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
- Coverage   99.02%   99.02%   -0.01%     
==========================================
  Files          85       85              
  Lines        6044     6040       -4     
==========================================
- Hits         5985     5981       -4     
  Misses         59       59              

Files	Coverage Δ
src/sfc-writer.c	98.70% <100.00%> (-0.01%)	⬇️

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