variant_storage upgrades:

[malachib]

1. Create a sparse mode awareness, like tuple/evaporator code has
2. Upgrade instance_storage to take full gamut of constructor inputs
3. Consider upgrading these to detail namespace or similar, used a lot and very useful overall

Also potential bug:

1. visitor_storage application of is_trivial sometimes gets Functors wrong and thinks they are trivial. Seen on GCC 12 x64