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remove GodotObject protocol (migueldeicaza#525)

It doesn't seem like the GodotObject protocol has any reason to exist.
This patch removes it.

-   The protocol has no requirements.

-   There is exactly one directly conforming type: Object. All
    descendants of Object inherit the conformance, but any constraint
    requiring GodotObject conformance can be rewritten to instead
    require Object inheritance.

-   There are three functions with GodotObject constraints:

    -   `VariantStorable.makeOrUnwrap`
    -   `Variant.asObject`
    -   `lookupObject`

    All test cases pass if I change these constraints to require Object
    inheritance instead of GodotObject conformance.

Co-authored-by: Rob Mayoff <mayoff@dqd.com>

remove GodotObject protocol (migueldeicaza#525)

It doesn't seem like the GodotObject protocol has any reason to exist.
This patch removes it.

-   The protocol has no requirements.

-   There is exactly one directly conforming type: Object. All
    descendants of Object inherit the conformance, but any constraint
    requiring GodotObject conformance can be rewritten to instead
    require Object inheritance.

-   There are three functions with GodotObject constraints:

    -   `VariantStorable.makeOrUnwrap`
    -   `Variant.asObject`
    -   `lookupObject`

    All test cases pass if I change these constraints to require Object
    inheritance instead of GodotObject conformance.

Co-authored-by: Rob Mayoff <mayoff@dqd.com>

Drop the `value:` parameter name from the various PackedArray.* append

methods.  Do that, but still provide a deprecated entry point to
prevent breaking code.

In a few cases where we surfaced an API that did the append but with
the underlying type, we keep the value: parameter in the API, but
introduce a convenience `append(_:)` method of the right type, for
example for PackedByteArray that would be:

'func append(_ value: UInt8)`

Only remove the callback value from our shared array when the last ex…

…ntesionLevel has been processed

Export enum (migueldeicaza#476)

Export enumeration support

Any enumeration that is blessed with `CaseIterable` can be then exported to Godot
by using `@Export(.enum)` on it, like this:

```
enum MyEnumeration: Int, CaseIterable {
    case first
    case second
}
```

To export, use the `.enum` parameter to Export:

```
@godot
class Demo: Node {
     @export(.enum)
     var myState: MyEnumeration
}
```

One limitation of the current change is that this works by using
`CaseIterable`, either by manually typing it, or using the
`PickerNameProvider` macro.

Document a case from Slack

Make @export work with GArray and Variant values, fixes migueldeicaza…

…#410

Add AStar engine tests (migueldeicaza#400)