WIP. Still need to do the same for DockerUses.

The idea here is to get rid of a few of the low-level allocations that happen over and over, namely in things like Uses parsing. The main trick here is using self_cell to hold onto the backing String (instead of destroying it through from_str).

We can actually go even further than this, since serde_yaml supports borrowing from the original input. But that'll be a bigger refactor, since we lose track of the original loaded input at the moment.