This will reduce noise in other PRs.

Test against Go 1.19 and lint only for the latest version.
Update to latest staticcheck to work with 1.19.

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.

Adds a Pointer[T] type that provide the same functionality as
Go 1.19's `sync/atomic.Pointer[T]` in both, Go 1.18 and 1.19.

In Go 1.19, this uses the standard library's implementation,
and in 1.18 our own custom implementation based on UnsafePointer.
For Go 1.17, even though it's now unsupported, this is a no-op.

Refs GO-1572
Resolves #115

Prepare a new release with the following changes:
v1.9.0...2ab21d8

This moves the templates used by gen-atomicint and gen-atomicwrapper
into external files embedded into the corresponding gen binary with
go:embed.

* Implement String method for pointers

By implementing this method ourselves using atomic
loading and fmt.Sprint, we can avoid possible unguarded
accesses to the embedded pointer if someone attempts to
fmt.Print the atomic.Pointer
.

* Use Pointer.Load, not wrapped.Load

Co-authored-by: Abhinav Gupta <[email protected]>

* Regenerate code to update copyright end year to 2023

* Test behaviour of default values initialized in different ways

This adds repro tests for #126 and #129

* Fix Swap and CompareAndSwap for Value wrappers

Fixes #126, #129

All atomic types can be used without initialization, e.g., `var v
<AtomicType>`. This works fine for integer types as the initialized
value of 0 matches the default value for the user-facing type.  However,
for Value wrappers, they are initialized to `nil`, which is a value that
can't be set (triggers a panic) so the default value for the user-facing
type is forced to be stored as a different value. This leads to multiple
possible values representing the default user-facing type.

E.g., an `atomic.String` with value `""` may be represented by the
underlying atomic as either `nil`, or `""`. This causes issues when we
don't handle the `nil` value correctly, causing to panics in `Swap` and
incorrectly not swapping values in `CompareAndSwap`.

This change fixes the above issues by:
 * Requiring `pack` and `unpack` function in gen-atomicwrapper as the
   only place we weren't supplying them was for `String`, and the
   branching adds unnecessary complexity, especially with added `nil`
   handling.
 * Extending `CompareAndSwap` for `Value` wrappers to try an additional
   `CompareAndSwap(nil, <new>)` only if the original `CompareAndSwap`
   fails and the old value is the zero value.

Set only read permission on CI workflows since they don't need write access.

Fixes #132.