* 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.
* Add atomic.Time
* fix the arg variable name in the generated code from x to t
* fix build err
* lint
* add changelog entry
* add more tests to show we preserve the whole time.Time struct
* removing add/sub/round
* code review feedback
