Add support for randomness customization

[Caellian]

Instead of forcing Pcg64Mcg. This PR adds ability to pass in a custom random generator for each model component (Generator, Excl. Gateway, Stochastic Gate, Processor), if None is provided a global rng will be used as previously.

I'm using Rc<RefCell<dyn Debug + RngCore>> for storage which is required because of possible type variance of rng's in Model structs (generics wouldn't work; requires variadic generics). As a side effect, this allows multiple Models to share the same rng.

I tested a simulation using this change and it didn't break. Not sure about performance impact, but simulation performance should be a bit worse because of the added jump and vtable lookup each time rng was previously used.
Let me know whether there's anything you'd like to change/improve in what I've done in this PR so far, or it's okay as it is.

Before merging I'd like to rename uniform_rng.rs to something else, or possibly move its contents to parent mod.rs. I'd appreciate your opinion on this.

Maybe unwanted changes

• Services struct now skips deserializing the randomness source as well (and constructs the default).
  • Given that any RngCore struct can be the global rng doesn't make sense to deserialize it anyway. Manually calling the (currently non-existant) rng setter seems like a better option.

[ndebuhr]

This is great, thanks @Caellian. Sorry for the delayed response. Merged, with test/clippy regressions fixed and uniform_rng renamed to dynamic_rng.