Store tools as a set of strings

[aieri]

As noted in get_stored_tools(), StoredState cannot store arbitrary objects; this commit applies the same logic to _on_redetect_hardware()

Note: it would be nice to turn stored_tools into a property, but that requires a lot of unit test updates.

Fixes: #364

[jneo8]

The type stored in _stored must be an instance of StoredObject, as outlined in the source code.

Since we're using the HWTool object as our model, we need to convert it into a set of strings. This conversion ensures it can be serialized and properly stored in SQLite.

[chanchiwai-ray]

Maybe it's worth considering having a update_stored_tool wrapper function, since this line is used in two places

[aieri]

Maybe it's worth considering having a update_stored_tool wrapper function, since this line is used in two places

yes, that's what led me to try to turn get_stored_tools() into the getter of a stored_tools property (with self._stored.stored_tools = {tool.value for tool in available_tools} being the setter). Two hours and 100+ lines of unit test changes later and I'm not yet done :'(
I find this charm's tests to be very hard to understand.