Adds CompareAndSwap and Swap methods to String, Error, and Value,
implemented by making use of Value.CompareAndSwap and Value.Swap
added in Go 1.17.
Following that, add CompareAndSwap to all other types with "CAS" methods
and deprecate CAS in favor of CompareAndSwap, since that's the convention
the standard library chose for these in Go 1.19.
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/
Add support for atomic `uintptr` and `unsafe.Pointer` types.
For `unsafe.Pointer`, name the atomic variant `atomic.UnsafePointer` to
maintain the "unsafe" portion of the name in usage.
Resolves#88
