Add the concept of a PeekType to Traits<T>. This is the type we'll
return (wrapped in an Optional) from HashMap::get().
The PeekType for OwnPtr<T> and NonnullOwnPtr<T> is const T*,
which means that HashMap::get() will return an Optional<const T*> for
maps-of-those.
Instead of aborting the program when we hit an assertion, just print a
message and keep going.
This allows us to write tests that provoke assertions on purpose.
Get rid of the ConstIterator classes for these containers and use templated
FooIterator<T, ...> and FooIterator<const T, ...> helpers.
This makes the HashTable class a lot easier to read.
