cudax::*_memory_resource should be an object not a type

[brycelelbach]

cudax::*_memory_resource should be a constexpr/constinst global object, not a type. I want to write foo(cudax::device_memory_resource) not foo(cudax::device_memory_resource{}). This is the common case, so we should optimize for it; if you need flags, you use the type name (which could be cudax::*_memory_resource_type, cudax::*_memory_resource_t, or something like that).

In Thrust, I believe we made these objects not types.

[miscco]

We considered adding global objects for those types, but decided against that because that has much stronger implications on stability of a type. Changing anything in the default object might break user code in an unclear and hard to debug way, which is why we opted out of providing global objects.