Tracking issue for adding partial_cmp to PartialOrd

[alexcrichton]

Tracking issue for rust-lang/rfcs#100.

Nominating.

[sfackler]

First bug found! https://github.com/rust-lang/rust/blob/master/src/libcollections/vec.rs#L383

[sfackler]

TreeMap as well: https://github.com/rust-lang/rust/blob/master/src/libcollections/treemap.rs#L73

[brson]

1.0, P-backcompat-libs.

[sfackler]

#15030