Auto merge of rust-lang#102698 - michaelwoerister:unord-collections, …

…r=lncr

Introduce UnordMap, UnordSet, and UnordBag (MCP 533)

This is the start of implementing [MCP 533](rust-lang/compiler-team#533).

I followed `@eddyb's` suggestion of naming the collection types `Unord(Map/Set/Bag)` which is a bit easier to type than `Unordered(Map/Set/Bag)`

r? `@eddyb`