Optimize string_to_array for scalar args #21129
Description
Is your feature request related to a problem or challenge?
string_to_array is typically invoked with scalar delimiter and null_str arguments. If we specialized the implementation for this case, we could improve efficiency (e.g., by using memchr::memmem::Finder to search for the delimiter more quickly).
Describe the solution you'd like
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Describe alternatives you've considered
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Additional context
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