Strip Value from to_output() function for GraphQL scalars

[tyranron]

Related to #1327

Synopsis

Counterpart of #1327 making the to_output() function returning ScalarValue directly instead of Value.

Solution

Now to_output() function should return a ScalarValue directly, instead of a Value.

Of course, the output type polymorphism is supported.

Either concrete type could be specified:

#[derive(Debug, GraphQLScalar)]
#[graphql(parse_token(String))]
struct TestComplexScalar;

impl TestComplexScalar {
    fn to_output(&self) -> &'static str {
        "SerializedValue"
    }
    // ...
}

Or a Displayable one:

#[graphql_scalar]
#[graphql(with = local_date, parse_token(String))]
pub type LocalDate = chrono::NaiveDate;

mod local_date {
    use std::fmt::Display;
    use super::LocalDate;

    pub(super) fn to_output(v: &LocalDate) -> impl Display {
        v.format("%Y-%m-%d")
    }
    // ....
}

Or just a generic:

    #[graphql_scalar]
    #[graphql(
        to_output_with = to_output,
        parse_token(prelude::String, i32),
    )]
    type StringOrInt = StringOrIntScalar;

    fn to_output<S: ScalarValue>(v: &StringOrInt) -> S {
        match v {
            StringOrInt::String(s) => S::from_displayable(s),
            StringOrInt::Int(i) => (*i).into(),
        }
    }
    // ...