fix(rust, python): don't check sortedness in asof by

[ritchie46]

fixes #8875

[mkleinbort-ic]

Sorry, for clarification - does this still check if the items are sorted within the groups?

[ritchie46]

No, it doesn't. That would be O(n), I am not sure how we could elide that

[mkleinbort-ic]

Mixed feelings about this - especially if we fail on unsorted keys when there is no by parameter, but possibly silently compute the wrong result if the by parameter is used.

I would favor the O(n) cost (checking an array is sorted is not that bad), at least as a default optional

df.join_asof(df2, ...., assume_sorted_keys=False)

For the subset of users that do need the extra milliseconds they can set assume_sorted_keys=True to skip the O(n) check.