Allow kernel_generator to be stateful

[oleksandr-pavlyk]

In python kernel generator is a user-defined callable. We need to capture Python object of that callable in kernel generator provided for each benchmark.

To this end, nvbench::benchmark has been modified to have member of kernel_generator type (must be copy-constructable). Constructor acquires an optional parameter of type kernel_generator with default value of default-contstructed instance.

nvbench::runner was modified to store kernel_generator instance as well. Its run method creates a fresh copy of stored instance for each invocation, just as it was happening before.

nvbench tests/examples pass with this change.