Consider replacing std::sync::Arc usages with an Arc implementation that has no weak support

[Veykril]

We use Arcs a lot, but never make use of the weak count so it might be beneficial to use a slimmer Arc (for our attribute handling it might even be more beneficial to employ a ThinArc).

[lnicola]

A prototype of this (which doesn't include the Chalk database part because it's external) seems to save a whooping 2 MB 4 MB of RAM.