CLN: fix core/algorithms.py dtype detection

[jreback]

after this is merged: #15299

we have separate if/else detections for pretty much every routines, e.g. things like

if needs_i8_conversion:
    ....
elif .....

This is very smelly and needs to be refactored

#15902 instroduces _ensure_data to do this, but is restricted ATM to nlargest/nsamllest and needs integration.

Note that this is actually a bit tricky, because we actually want to return a bit more data, including a previously localized tz (for re-localization after).

[chris-b1]

Maybe _ensure_data could also return a reconstruction function to handle the metadata. Something like

def unique(arr):
    arr, reconstructor = _ensure_data_view(arr)
    # labels, uniques = ....
    return labels, reconstructor(uniques) #handles Index, tz, Categorical