Add ScalarValue::downcast_type() method for GraphQLScalar parsing optimization (#819)

[tyranron]

Part of #819
Continued from #1324

Synopsis

As a counterpart for the ScalarValue::from_displayable() allowing optimizing ScalarValue creation from arbitrary types, this PR introduces a ScalarValue::downcast_type() method, which allows checking whether a generic ScalarValue already represents the desired type:

#[derive(GraphQLScalar)]
#[graphql(from_input_with = Self::from_input, transparent)]
struct Name(ArcStr);

impl Name {
    fn from_input<S: ScalarValue>(
        v: &Scalar<S>,
    ) -> Result<Self, <S as TryScalarValueTo<'_, &str>>::Error> {
        // Check if our `ScalarValue` is represented by an `ArcStr` already, and if so, 
        // do the cheap `Clone` instead of allocating a new `ArcStr` in its `From<&str>` 
        // implementation.
        if let Some(s) = v.downcast_type::<ArcStr>() {
             Ok(Self(s.clone()))
        } else {
            v.try_to::<&str>().map(|s| Self(s.into()))
        }
    }
}