[Skeletal Model] Support no_std + alloc in skeletal model

[TheButlah]

There is a decent chance that someone will want to try running the skeletal model on a microcontroller without an OS. We should support a no_std + alloc environment.

Current blockers:

• [ ] we use thiserror and that doesn't support no_std. Could try snafu instead.
• [ ] petgraph doesn't support no_std 🥲