Allow adding metadata to declarations

[jakemac53]

Today you cannot add additional metadata (or macros) to existing declarations, but we may want to allow you to do so. You can add both of these when creating new declarations.

We would need to flesh out the rules around this, and how it would interact with https://github.com/jakemac53/macro_prototype/issues/7.

[goderbauer]

Having this would be useful. In the autodispose example it seems strange that the annotation has to move from the class to the dispose method, if one is present. With this, I imagine you could always just annotate the class and the class macro could add the autodispose annotation to the existing dispose method, if necessary, to add the additional logic there.