add tests for `min` and `max` from `Float`
stats: fix handling of NaN in `min` and `max`

The `cmp::min` and `cmp::max` functions are not correct with partially
ordered values. #12712
auto merge of #12718 : thestinger/rust/min_max, r=alexcrichton
Merge branch 'master' of https://github.com/mozilla/rust