Start on GPU extensions

[kshyatt]

Lots of stuff still broken but some things are working

[kshyatt]

cc @Jutho and @lkdvos

Where we're at now:

• Most tests are passing for CUDA except for the @constinferred (haven't tried super recently) and a correctness test in rightorth with Polar
• CUDA extension is currently using CUDA.DeviceMemory hardcoded because this dramatically simplifies getting all the types to play nicely. My goal is to get to a "minimum passing extension" for the tests and then experiment with what can be simplified and stripped out.
• Once I get the CUDA extension shaped up and working nicely, I'll try to graft as much as I can to AMD.
• We're still waiting on some PRs I made to CUDA.jl itself to fix interface issues to merge and for a new version to tag, that will also help with tests passing.