Snapshot metadata

This isnt for merging yet, just WIP,

Bit of a slow start, but figured while I am still finding my way around the codebase i'll raise this so ppl can peek at the progress more conveniently and give me feedback as i go.
Shouldn't need the unsafe. Macro should take care of it with `KeyName(".allocated")` (if it doesn't there is a bug / missing code elsewhere).