Benchmarks: Simplify implementation of `bench_send_on_m_of_n()`
Benchmarks: Simplify implementation of `bench_send_on_m_of_n()`

Various small changes to make the code simpler, while preserving
functionality.

There is indeed one minor functional change though: this version no
longer actually iterates the results vector. In theory, this should have
a minor performance impact -- however, I expect it to be insignificant.

servo/ipc-channel