Make Expr::Interval its own struct

[aprimadi]

Currently Expr::Interval is part of Expr enum. Instead, we should create its own struct because it has a lot of fields.

pub enum Expr {
    ...

    /// INTERVAL literals, roughly in the following format:
    /// `INTERVAL '<value>' [ <leading_field> [ (<leading_precision>) ] ]
    /// [ TO <last_field> [ (<fractional_seconds_precision>) ] ]`,
    /// e.g. `INTERVAL '123:45.67' MINUTE(3) TO SECOND(2)`.
    ///
    /// The parser does not validate the `<value>`, nor does it ensure
    /// that the `<leading_field>` units >= the units in `<last_field>`,
    /// so the user will have to reject intervals like `HOUR TO YEAR`.
    Interval {
        value: Box<Expr>,
        leading_field: Option<DateTimeField>,
        leading_precision: Option<u64>,
        last_field: Option<DateTimeField>,
        /// The seconds precision can be specified in SQL source as
        /// `INTERVAL '__' SECOND(_, x)` (in which case the `leading_field`
        /// will be `Second` and the `last_field` will be `None`),
        /// or as `__ TO SECOND(x)`.
        fractional_seconds_precision: Option<u64>,
    },

    ...
}

I think it is better this way:

pub enum Expr {
    ...

    /// INTERVAL expression
    Interval(Interval),

    ...
}

[alamb]

Makes sense to me -- thank you @aprimadi -- I plan to make a new release of sqlparser sometime over the next week or so #847 -- maybe we can get this into it too