Room list visibility cache includes event data

[alphapapa]

I just realized that the persisted value of ement-room-list-visibility-cache includes room event data. On my account, that serialized to over 150 MB (which could explain some apparent slowness for some users at times).

Fixing this requires that we define a subclass of magit-section to insert our sections as, and then an ident value function to return simple identifiers (e.g. room ID-session ID pairs).

Then we must pass that subclass to taxy-magit-section-insert. That will require a new version of taxy-magit-section, but it's a simple change.

[alphapapa]

See alphapapa/taxy.el@f43bbf6

[alphapapa]

I've pushed taxy-magit-section v0.13. It should appear on ELPA in the next day. Then this can be merged.