More accurate speed benchmark

More accurate speed benchmark

Used smaller buffers to minimise the impact of cache misses in the
benchmark.  Chosen a size that makes Libsodium and Monocypher look best.
(There is a trade-off between start up time and throughput.)

This should highlight the algorithmic differences better.  Still, the
memory access patterns are very clean, computation tends to dominate.
Ultimately, this makes little difference.