Move Value::Interval to Expr::Interval

[ding-young]

Current ast Interval is defined as a variant of enum Value, which means a literal value. However, considering that the field value of Interval holds wrapped Expr, Value becomes somewhat requires endless evaluation... not "literal". Personally, it would be nice to change Value::Interval to Expr::Interval.

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>,
    }

[alamb]

This makes sense to me @ding-young -- thank you for the contribution

I think @togami2864 added Expr support for Interval in #517 whereas it previously was only a literal

[coveralls]

Pull Request Test Coverage Report for Build 3099121905

• 33 of 41 (80.49%) changed or added relevant lines in 3 files are covered.
• 100 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.05%) to 85.609%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	3	4	75.0%
src/ast/mod.rs	21	28	75.0%

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_common.rs	10	97.0%
src/parser.rs	90	83.27%

Totals
Change from base Build 3019512499:	0.05%
Covered Lines:	9768
Relevant Lines:	11410

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💛 - Coveralls