/// Provides means to map enum values to positions in arrays backing an EnumMap/EnumTable.
/// Not intended to be implemented by hand. Deriving it with `#[derive(Enumerated)]`
/// attribute macro is preferred.
///
/// ```
/// use enum_collections::Enumerated;
/// #[derive(Enumerated)]
/// enum Letter {
/// A,
/// B,
/// }
///
/// assert_eq!(Letter::SIZE, 2);
/// ```