Replace Value enum with dyn QueryValue

[tqwewe]

This PR deprecated the old Value enum, and creates a new struct: Value(Box<dyn QueryValue>).

Any type that implements the new trait QueryValue can be used as a Value.

/// Indicates that a type is supported for use in SQL queries.
pub trait QueryValue: Clone + PartialEq {
    /// Returns the value as an escaped string safe for use in SQL queries.
    fn query_value(&self, query_builder: &dyn QueryBuilder) -> String;
}

Many of the primitive types implement QueryValue, including:
&str, String, i8, i16, i32, i64, u8, u16, u32, u64, f32, f64, bool, () (represents NULL), Vec<u8>, serde_json::Value, chrono::NaiveDate, chrono::NaiveTime, chrono::NaiveDateTime, chrono::DateTime<chrono::FixedOffset>, uuid::Uuid, rust_decimal::Decimal, bigdecimal::BigDecimal.

This means they can be used in queries (as they would previously), and calling .into() where necessary.

The old enum Value has been renamed to PrimitiveValue and has been marked as deprecated. This PR updates sea-query's version to 0.17.0. If this needs to be changed, let me know :)

[tyt2y3]

Thank you for the effort.

There seems to be a lot of useful ideas!

1. ().into()
2. the Default on ColumnSpec
3. V: Into<Value> on Update::value

And the impact to users is seemingly small.

My only wonder is, what new possibilities does this change bring to users?

[tqwewe]

My only wonder is, what new possibilities does this change bring to users?

I finally can answer this question with the PR in Sea ORM SeaQL/sea-orm#214

It allows support for CAST($1 as type), which lets you cast into custom types, such as enums in Postgres.

[tqwewe]

I couldn't find a way to do this without .cloned(). I think some changes need to be made in impl<T> QueryValue for Option<T>, but I had issues getting it to work.

[tqwewe]

With the new issue made SeaQL/sea-orm#252, I wil close this PR for now. If needed it can be reopened.

On a side note, I feel over using dynamic trait objects is tempting but not always the solution.