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.
This can definitely be improved with better trivial type detection and
by using the TypedTransfer template in more places.
It's a bit annoying that we can't get <type_traits> in Vector.h since
it's included in the toolchain compilation before we have libstdc++.
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.
The SpinLock was all backwards and didn't actually work. Fixing it exposed
how wrong most of the locking here is.
I need to come up with a better granularity here.
