Add partial_min/max to libcore/cmp

Add partial_min/max to libcore/cmp

Match against None and mark as experimental

Shortened documentation.
auto merge of #17482 : hoeppnertill/rust/master, r=alexcrichton

Intended to prevent each user to write his own partial_min/max, possibly differing in slight details. @sfackler encouraged to PR this on IRC.

(Let's hope this works... First PR.)