`capptr_domesticate<Backend>(Backend::LocalState*, CapPtr<T, B>)` is the
intended affordance for conversion to covert from a `CapPtr<T, B>` with
`B::wildness` `Wild` to a `CapPtr<>` with `B::with_wildness<Tame>` and thence
plumbed into the rest of the machinery.
David added the SFINAE wrapper so that `Backend`-s now don't need to implement a
domestication callback; instead, if the `Backend` does not provide a
`capptr_domesticate` function, a default, which just does an explicit type cast,
will be used instead.
This is not yet hooked into the rest of the tree.
Co-authored-by: David Chisnall <David.Chisnall@microsoft.com>
David points out that we might not have a static way to get at the pagemap, so
it is potentially useful to pass pointers to state objects down from the
Allocators.
And do so by type, rather than by value. While here, introduce a C++20 concept
for this Backend-offered proxy and adjust the template parameters appropriately.
This will be useful for the process sandbox code, which needs to mediate stores
to the pagemap, but can provide a read-only view.
