Normally you want to access the T&, but sometimes you need to grab at
the NonnullPtr, for example when moving it out of the Vector before
a call to remove(). This is generally a weird pattern, but I don't have
a better solution at the moment.
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. :^)