Consider use of compact regions to reduce GC times

A large chunk of the data stored by consensus is the ledger state. A ledger state in principle is a single value; ideally it would be GCed and traversed as a whole, an obvious candidate for compact regions. There are some technical difficulties with this (pinned data for instance), but also some fundamental ones: we'd still like to maintain sharing between _multiple_ ledger states. Not an easy problem, but perhaps worth thinking about at some stage..

Possibly useful reading: https://tech.channable.com/posts/2020-04-07-lessons-in-managing-haskell-memory.html .