"'lifetime" gives no useful semantic information there. All 'somethings are lifetimes; it would be like naming a type parameter "Type," or a variable "variable." If you don't have a specific shard context that would give the name meaning, `'a` is as good as any. I do agree that something like Key or Value would probably be better than K or V.
Yes, in general, it is a good idea to give more descriptive names but in the context of a hashtable, K and V are good enough. I seriously doubt that there is anyone reading the source code of a hashtable and cannot figure out in milliseconds what K and V mean (how fast is the human brain?:).
