Add an "ElementType" typedef to NonnullOwnPtr and NonnullRefPtr to allow
clients to easily find the pointee type. Then use this to remove a template
argument from NonnullPtrVector. :^)
It's not possible to grow one of these vectors beyond what's already in them
since it's not possible to default-construct Nonnull{Own,Ref}Ptr.
Add Vector::shrink() which can be used when you want to shrink the Vector
and delete resize() from the specialized Vectors.
This works just like NonnullRefPtr, except for NonnullOwnPtr's instead.
NonnullOwnPtrVector<T> inherits from Vector<NonnullOwnPtr<T>>, and adds some
comforts on top, like making accessors return T& so we can chase dots (.)
instead of arrows (->) :^)
