Policy VM: Change Enum representation to be orderable

[apetkov-so]

We want to use enums in fact keys, and there shouldn't be any reason why you can't do that, except enum values are internally string pairs instead of integers (IIRC this was done to ease interface generation).

I think it would make more sense to indirect enums through a definition stored in the machine. The actual value stores a type name and an index into that definition. That index will be naturally orderable, then it can be used in fact keys.

[apetkov-so]

@chip-so, @jdygert-spok There's a macro, value, that tries to parse a string into an enum.

#[value]
#[derive(Default)]
pub enum TestEnum {
    #[default]
    A,
    B,
    C,
}

This now requires access to the RunState to lookup the enum value identifier. I'm not sure how we can make it work.

EDIT: Actually, I don't think it does. We can just use the value indices. Only enum validation requires access to the RunState.

[apetkov-so]

@chip-so @jdygert-spok Can I get some help with the value macro? I just can't figure out how to map the identifiers to their indices.