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 https://github.com/r-spatial/sf/issues/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 :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/paleolimbot/wk/pull/195?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Dewey+Dunnington) | Coverage Δ | | |---|---|---| | [src/sfc-writer.c](https://app.codecov.io/gh/paleolimbot/wk/pull/195?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Dewey+Dunnington#diff-c3JjL3NmYy13cml0ZXIuYw==) | `98.70% <100.00%> (-0.01%)` | :arrow_down: |

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