Replace `DRW_Attributes` with a VectorSet of std::string. The max number of
attributes is still the same. The inline buffer size is 4, and std::string's inline
buffer is smaller than the previous char array size of 64, but it seems
reasonable to save those optimizations for shorter attribute names and
fewer attributes. In return we significantly decrease the size of the batch
caches, simplify the code, and remove the attribute name length limit.
I observed roughly an 8% increase in the 30k cube objects file, a change from
12 to 13 FPS. I'm guessing this is mostly because `VectorSet<std::string>` is
smaller than `DRW_Attributes`.
Pull Request: https://projects.blender.org/blender/blender/pulls/138946
760cf70d63