Expose enum value as integer

[dtolnay]

#170 produces "enums" with a private integer representation, and public associated constants corresponding to the variants.

#[derive(Copy, Clone, PartialEq, Eq)]
#[repr(transparent)]
pub enum SmallPrime(u32);

#[allow(non_upper_case_globals)]
impl SmallPrime {
    pub const Two: Self = SmallPrime(2);
    pub const Three: Self = SmallPrime(3);
    pub const Five: Self = SmallPrime(6);
}

This behaves like an ordinary Rust enum in all ways except that it enforces a wildcard match arm when matched (which is deliberate) and that it cannot be cast using as to a primitive (which is unfortunate).

We should expose some way to get the primitive value. Maybe a method, maybe just a straightforward public field like n.repr.