This change is a first step to make the SAPI variable hierarchy more sensible.
It turns the `Reg<T>` class into a descendant of `Pointable`, but without
making its `PtrXXX()` methods public (hence the `using` statements). Further
changes are needed to restructure this.
There are no functional changes and the class sizes, including vtables, should
not change.
PiperOrigin-RevId: 413333120
Change-Id: I90ceeaeb7aea482016f8f4bee81489d5a9db9ade
