When hashing something that ends up affecting properties like symbol names, we want these symbol names to be calculated independently of other factors like what architecture you're compiling from.
A vector container that makes sure that its items are hashed in a stable order.
Something that implements
Implement this for types that can be turned into stable keys like, for example, for DefId that can be converted to a DefPathHash. This is used for bringing maps into a predictable order before hashing them.