This change is a renaming with no functional changes.
It includes the following renames:
* `val` for arguments that replace the atomic value (e.g., `Store`).
* `delta` for arguments that offset the atomic value (e.g., `Add`).
* `old`, `new` for arguments to `CAS`.
* `old` named return from `Swap`.
* `swapped` for named return from `CAS`.
This also matches the names used in the stdlib atomic interface:
https://golang.org/pkg/sync/atomic/
d8a8972198