(GPU Testing Part 4) New GPU test utilities and example.

This introduces three new functions:

// returns kernel(args...) running on the CPU.
Eigen::run_on_cpu(Kernel kernel, Args&&... args);
    
// returns kernel(args...) running on the GPU.
Eigen::run_on_gpu(Kernel kernel, Args&&... args);
Eigen::run_on_gpu_with_hint(size_t buffer_capacity_hint, Kernel kernel, Args&&... args);
    
// returns kernel(args...) running on the GPU if using
//   a GPU compiler, or CPU otherwise.
Eigen::run(Kernel kernel, Args&&... args);
Eigen::run_with_hint(size_t buffer_capacity_hint, Kernel kernel, Args&&... args);

Running on the GPU is accomplished by:

• Serializing the kernel inputs on the CPU
• Transferring the inputs to the GPU
• Passing the kernel and serialized inputs to a GPU kernel
• Deserializing the inputs on the GPU
• Running kernel(inputs...) on the GPU
• Serializing all output parameters and the return value
• Transferring the serialized outputs back to the CPU
• Deserializing the outputs and return value on the CPU
• Returning the deserialized return value

All inputs must be serializable (currently POD types, Eigen::Matrix and Eigen::Array). The kernel must also be POD (though usually contains no actual data).

Tested on CUDA 9.1, 10.2, 11.3, with g++-6, g++-8, g++-10 respectively.

This MR depends on !622 (merged), !623 (merged), !624 (merged).