Refactor `CUDF` typescript exports

[Matthew Zegar]

CUDF currently uses an old way to export .cpp types which involves creating and exporting interfaces. For example...

export interface Table {...

export const Table: TableConstructor = CUDF.Table;

in table.ts

We can avoid doing these exports by following an export method currently done by our CUDA library. Consider the following files noda_cuda.ts cuda/src/addon.ts. These files use a different way to export the .cpp types. We should do the same for CUDF.