When T in HashTable<T> has a potentially slow equality check, it can be
very profitable to check for a matching hash before full equality.
This patch adds may_have_slow_equality_check() to AK::Traits and
defaults it to true. For trivial types (pointers, integers, etc) we
default it to false. This means we skip the hash check when the equality
check would be a single-CPU-word compare anyway.
This synergizes really well with things like HashMap<String, V> where
collisions previously meant we may have to churn through multiple O(n)
equality checks.
It is a non-null `T*` with reference semantics.
Or a `T&` whose address can be copied and re-assigned.
Or a `NonnullRefPtr` whose memory is not managed.
It can be useful when you want to store a reference in a
data structure that needs to be copyable or assignable.
