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