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.
Something that implements
HashStable<CTX> can be hashed in a way that is
stable across multiple compilation sessions.
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.