Standard abstraction for specifying thread grid hierarchy and dimensions

[ericniebler]

CUDA C++ Runtime needs a thread hierarchy description object that specifies a mix of static and dynamic dimensions. Hierarchy will consist of the fundamental block and grid level and allow for optional levels like cluster or custom levels. It will also be expandable with future levels.
There will also be a collection of queries available in both host and device code to count, index etc the entities in the hierarchy and extract the compile-time information from it.

This is a fundamental building block for #2038, but is meant to be reusable outside of that API.

Tasks

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1. Add hierarchy type and queries
2. Add meta dimensions
3. Provide configuration options for levels
4. Add proof of hierarchy validation for device side optimizations