Make RawValue::from_borrowed public

[matthiasg]

I would propose making from_borrowed public. It is very helpful in certain scenarios where a string is known to be stored in a structure which then needs to be serialized as is.

Theoretically this could also be handled by a serde field macro hint e.g #[serde(raw)] etc, but it is also helpful in other contexts.

Is there a specific reason of making this not public ?