Generic element model.

As deserialization differs slightly between FHIR releases, Element is generic over a FHIR release.

Example

use fhirbolt::FhirReleases;
use fhirbolt::element::{Element, Value, Primitive};

let mut element = Element::<{ FhirReleases:: R4B }>::new();
element.insert(
"resourceType".to_string(),
Value::Primitive(
Primitive::String("Observation".to_string())
)
);
// ...