The function min_set returns a Vec of all the minimum values. All
variants for max and with key extraction and comparison functions are
added also.

Since the functions need to return an unknown number of values and the
values are not known until the iterator is finished, the function
needs to allocate memory. Therefore Vec is used for returning the
values.

…n<Vec>

…alOrd

We may relax this bound at some point, but I'd go with this until we
have evidence that it bothers users, as there have been cases where I
 actually was happy to be informed about Ord vs PartialOrd.

We may relax this bound at some point, but I'd go with this until we
have evidence that it bothers users, as there have been cases where I
 actually was happy to be informed about Ord vs PartialOrd.

… instead of bool

More canonical.

Slightly simplifies control flow as it avoids mutable variables that may
not even be used in case of early return.

the {min,max}_set results must contain the result of the corresponding
{min,max} variant.