Add benchmark for OT extension

Comparison of multiplexed versus non-multiplexed:

    $ go test -cpu 4 -bench . github.com/tjim/smpcc/runtime/ot
    testing: warning: no tests to run
    PASS
    Benchmark2-4	       1	82873401453 ns/op
    Benchmark1-4	       1	6346259208 ns/op
    ok  	github.com/tjim/smpcc/runtime/ot	89.224s

Multiplexed is Benchmark1; both running the same number of sends, looks
like a good improvement.