This is just like OwnPtr (also single-owner), except it cannot be null.
NonnullOwnPtr is perfect as the return type of functions that never need to
return nullptr.
It's also useful as an argument type to encode the fact that the argument
must not be nullptr.
The make<Foo>() helper is changed to return NonnullOwnPtr<Foo>.
Note: You can move() out of a NonnullOwnPtr, and after that the object is
in an invalid state. Internally it will be a nullptr at this point, so we'll
still catch misuse, but the only thing that should be done in this state
is running the destructor. I've used consumable annotations to generate some
warnings when using a NonnullOwnPtr after moving from it, but these only
work when compiling with clang, so be aware of that.
We shouldn't allow constructing e.g an OwnPtr from a RefPtr, and similar
conversions. Instead just delete those functions so the compiler whines
loudly if you try to use them.
This patch also deletes constructing OwnPtr from a WeakPtr, even though
that *may* be a valid thing to do, it's sufficiently weird that we can
make the client jump through some hoops if he really wants it. :^)
This is prep work for supporting HashMap with NonnullRefPtr<T> as values.
It's currently not possible because many HashTable functions require being
able to default-construct the value type.
Also run it across the whole tree to get everything using the One True Style.
We don't yet run this in an automated fashion as it's a little slow, but
there is a snippet to do so in makeall.sh.
