Function parse 

pub fn parse(src: &str) -> Result<Value, TabnasError>

Parse one document with the default operator table.

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let value = tabnas_expr::parse("a:1+2")?;
    assert_eq!(tabnas_expr::simplify(&value).to_string(),
               r#"{"a":["+",1,2]}"#);
    Ok(())
}