I first re-named the existing method to `min_unchecked` and made it
internal to the crate.

I then added a `pub min(&self) -> Opiton<&T>` method.

I figure we can first change the public API before deciding what to do
about internal usage.

Ref: apache#6093

I first re-named the existing method to `max_unchecked` and made it
internal to the crate.

I then added a `pub max(&self) -> Opiton<&T>` method.

I figure we can first change the public API before deciding what to do
about internal usage.

Ref: apache#6093

Ref: apache#6093

This removes ambiguity around whether the between all values are non-null or just that the null count stat is missing

Ref: apache#6215

Changing null_count from u64 to Option<u64> increases the memory size and layout of the metadata.

I included these tests as a separate commit to call extra attention to it.

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Per PR review, we will deprecate the old API instead of introducing a brekaing change.

Ref: apache#6216 (review)

This adds the API and migrates all of the test usage.
The old APIs will be deprecated next.

The check is unnecessary now that the stats funcs return Option<T> when unset.

An internal version was also created because it is used so extensively in testing.

…s_nulls

This removes the assertion from any test that subsequently unwraps both
min_opt and max_opt.

…th assertions on min_opt and max_opt

This removes all use of Statistics::_internal_has_min_max_set from the code base, and so it is also removed.