Auto merge of rust-lang#105175 - michaelwoerister:add-stable-ord-trait, r=nagisa

Add StableOrd trait as proposed in MCP 533.

The `StableOrd` trait can be used to mark types as having a stable sort order across compilation sessions. Collections that sort their items in a stable way can safely implement HashStable by hashing items in sort order.

See rust-lang/compiler-team#533 for more information.

Author: bors