interp: add first Run benchmark
interp: add first Run benchmark

For now, just benchmarking some basic functions.

name   time/op
Run-4  1.50ms ± 1%

name   alloc/op
Run-4   165kB ± 0%

name   allocs/op
Run-4     893 ± 0%

mvdan/sh,mvdan/sh,mvdan/sh,mvdan/sh