Add as_raw to windows-registry

[kennykerr]

This returns the underlying registry key handle for interop purposes.

Fixes: #2999

[tim-weis]

This duplicates the key handle but Key also implements Drop potentially leaving callers with an invalid handle. Should this be unsafe?

Alternatively, this could be pub fn into_inner(self) -> isize instead making the transfer of ownership obvious.

[kennykerr]

There is no transfer of ownership here. This is similar to https://doc.rust-lang.org/stable/std/vec/struct.Vec.html#method.as_ptr where the underlying value is returned without changing ownership and requiring the caller to use it judiciously.