move stdcopy to pkg/stdcopy

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
stdcopy: improve perf by avoiding buffer growth

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Benchmark for StdWriter.Write

Signed-off-by: Alexandr Morozov <lk4d4@docker.com>
Merge pull request #6 from LK4D4/benchmark_for_8084

Benchmark for StdWriter.Write
Merge pull request #8084 from unclejack/improve_stdcopy

stdwriter: improve perf by avoiding buffer growth