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

`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.