It was a bit odd that you could create a Userspace<int> and that
Userspace<int>::ptr() returned an int instead of an int*.
Let's use C++20 concepts to only allow creating Userspace objects with
pointer types. :^)
Since we already have the type information in the Userspace template,
it was a bit silly to cast manually everywhere. Just add a sufficiently
scary-sounding getter for a typed pointer.
Thanks @alimpfard for pointing out that I was being silly with tossing
out the type.
In the future we may want to make this API non-public as well.
This will be used in the kernel to wrap pointers into userspace memory
without convenient direct access. The idea is to use the compiler to
enforce that we don't dereference userspace pointers.
