Properly Define What is the *top level* Declaration in Canopy

[isVoid]

Canopy is picky about declarations. Currently it only parses a subset of all declarations from the CUDA C++ declarations. It will ignore declarations that are:

1. Private / Protected methods / attributes in a class
2. Declarations that are not in the specified list of paths to keep passed from API.

For 1), this is because Canopy currently should only parse declarations that CUDA C++ developers intend to expose to users. Extending this idea a bit, we should probably also extract namespace information and allow filtering by namespace (e.g. namespace detail) and function names (e.g. _internal_func).