Specialize `PartialOrd` for totally ordered primitive types
Specialize `PartialOrd` for totally ordered primitive types

Knowing the result of equality comparison can enable additional
optimizations in LLVM.

Additionally, this makes it obvious that `partial_cmp` on totally
ordered types cannot return `None`.

aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust