This implements Optional<T&> as a T*, whose presence has been missing
since the early days of Optional.
As a lot of find_foo() APIs return an Optional<T> which imposes a
pointless copy on the underlying value, and can sometimes be very
misleading, with this change, those APIs can return Optional<T&>.
This avoids a value copy when calling value() or value_or() on a
temporary Optional. This is very common when using the HashMap::get()
API like this:
auto value = hash_map.get(key).value_or(fallback_value);
This patch introduces a new operator== to compare an Optional to its
contained type directly. If the Optional does not contain a value, the
comparison will always return false.
This also adds a test case for the new behavior as well as comparison
between Optional objects themselves.
