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:

pluggy/src/pluggy/callers.py

Lines 195 to 198 in a5130ac

	if firstresult: # first result hooks return a single value
	outcome = _Result(results[0] if results else None, excinfo)
	else:
	outcome = _Result(results, excinfo)

[wahuneke]

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