take FnMut instead of Fn

[m-mueller678]

Currently, the functions on Search take a impl Fn(T) -> Ordering when a impl FnMut(T) -> Ordering would suffice. I realize that in most cases the comparator should not need exclusive access, but unfortunately in my case due to a complicated generics situation I cannot implement Fn. Taking FnMut would also be in line with binary_search_by from std