fix: add missing `AtomicOperation.mutate()` types

[vwkd]

This adds the missing AtomicOperation.mutate() type. Also, it moves the referenced KvMutation type to the same file and adds missing docs.

Not sure if this is right. The prose for the docs are copied from the official docs for KvMutation and AtomicOperation.mutate(). However, there's no exported type KvU64 so can't link to it with {@linkcode KvU64} - similar references in other methods like sum seem to be removed. Also, couldn't figure out how to replicate the same prose wrap formatting.

Fixes #121.