Make stable hashing names consistent (part 1) This PR starts the implementation of MCP 893. It renames the things that appear in the `HashStable` trait, changing this: ``` pub trait HashStable { fn hash_stable<Hcx: HashStableContext>(&self, hcx: &mut Hcx, hasher: &mut StableHasher); } ``` to this: ``` pub trait StableHash { fn stable_hash<Hcx: StableHashCtxt>(&self, hcx: &mut Hcx, hasher: &mut StableHasher); } ``` Details in individual commits. This is the biggest part of the renaming. A follow-up PR will rename the remaining things. r? @jieyouxu