parallel: more efficient Run

We refactor Run to make it faster and more memory efficient.

Also added some benchmarks to verify this:

benchmark                old ns/op     new ns/op     delta
BenchmarkRunSingle       3163          1933          -38.89%
BenchmarkRun1000p100     714010        379910        -46.79%

benchmark                old allocs     new allocs     delta
BenchmarkRunSingle       10             3              -70.00%
BenchmarkRun1000p100     3007           3              -99.90%

benchmark                old bytes     new bytes     delta
BenchmarkRunSingle       412           240           -41.75%
BenchmarkRun1000p100     48394         240           -99.50%

Note that in almost all cases, the thing we're running will dwarf
the overhead of parallel.Runner. I'm much more interested
in the reduction in memory pressure this changes gives us.