Function rust_lisp::utils::require_string_parameter

pub fn require_string_parameter<'a>(
    func_name: &str, 
    args: &'a [Value], 
    index: usize
) -> Result<&'a str, RuntimeError>

Given a Value assumed to be a Value::List(), grab the item at index, assumed to be a Value::String(), and return a reference to its inner String. Err if any part of this fails.