fix: Rewrite implementation of top_k/bottom_k and fix a variety of bugs

[orlp]

This is a rewrite of our top_k and bottom_k implementations, to match our new well-defined behavior around nulls and lack of output order guarantee, and fixes a variety of bugs which were never reported, e.g.:

>>> pl.Series([1, 2, 3]).sort().bottom_k(1)
shape: (1,)
Series: '' [i64]
[
        3
]

>>> pl.Series([False, True]).bottom_k(5)
shape: (5,)
Series: '' [bool]
[
        false
        true
        true
        true
        true
]

Closes #16748.