NRT API Updates

[brandon-b-miller]

This PR makes a number of changes to the NRT API.

1. Memory allocations are no longer performed through the global MemSys object. It turns out due to technical limitations we can't call function pointers from a cuda module they weren't compiled in, so we can't have a single global MemSys holding malloc and free wrappers that we can safely access from everywhere the NRT API is used. Instead, the NRT APIs will just call malloc and free directly, rather than TheMSys.malloc(...).

2. The remainder of the MemSys object (which amounts to the stats counting component) has been changed to be stored as a pointer which can be set by an external function. The caller will have to separately allocate TheMSys and call the setter function before using any NRT APIs that use TheMSys if stats are to be recorded.

3. This PR adds the set of output functions that are needed to post-process a meminfo-tracked string array into a cudf string column including appropriately dereferencing output strings using the NRT API.