GlobalPointer pointer arthimetic

When I try to add an integer to a GlobalPointer!T type for pointer arithmetic, the resulting type becomes T*. The desired fix would be for the type to become GlobalPointer!T again after addition.

Currently, the code may look something like
```d
struct Pointer(AddrSpace as, T)
{
    T* ptr;
    alias ptr this;
}
```
Will need to add some operator overloads for opBinary!"+".
