tracing: store called hook_impls

[blueyed]

With tracing I think it would be good to know where the result is coming from (firstresult=True), and/or which hook_impls have been called in general.

But this could also be useful to know in general, so storing it (optionally?) on the _Result seems like a good idea to me. It should probably not be full references (except for when tracing is enabled, but that's not easy to know when creating the _Result it seems), but only the plugin_name by default.

Related code: https://github.com/pytest-dev/pluggy/blob/a5130ac87a71865192db3959d2e1e1ac41daaca7/src/pluggy/callers.py#L195-L198

[wahuneke]

Is this a dupe of #235 ? Commenting here in order to create a link between the issues