/// Helper macro for unwrapping `Option` values while returning early with an
/// error if the value of the expression is `None`. Can only be used in
/// functions that return `Option` because of the early return of `None` that
/// it provides.
///
/// # Examples
///
/// ```
/// #[macro_use]
/// extern crate try_opt;
///
/// use std::collections::HashMap;
///
/// fn map_add_checked(map: &HashMap<&str, i32>, lhs: &str, rhs: &str) -> Option<i32> {
/// let lhs = try_opt!(map.get(lhs));
/// let rhs = try_opt!(map.get(rhs));
/// lhs.checked_add(*rhs)
/// }
///
/// fn main() {
/// let mut map = HashMap::new();
/// map.insert("foo", 2);
/// map.insert("bar", 4);
/// map.insert("baz", 12);
/// assert_eq!(map_add_checked(&map, "foo", "bar"), Some(6));
/// assert_eq!(map_add_checked(&map, "baz", "qux"), None);
/// }
/// ```