Benchmark supports:
=> Looping on the GPU for a specified number of iterations
=> Looping infinitely on the GPU
=> Logging the sum time (ns and ms) for all iterations to complete

Also splits `testbench.h` out of `copy_testbench.h` for use in other
benchmarks.

Benchmark supports:
=> Generating a chain of copies in a single context
    - No timing pause between copies
=> Copies to, from, or between device(s)
=> Variable size and number of copies
=> Logging how many milliseconds each copy takes

*** Warning ***
* Relies on implicit synchronization of unidirectional copies across
* streams. Verify that this is still the case with CUDA before
* trusting results. One can check by verifying that the runtime for
* one iteration roughly matches the time for each iteration in a
* multi-iteration run.
***