os.proc on Scala Native

[szymon-rd]

Add support for working with processes for Scala Native. It seems to be blocked by lack of support for Threads on Native. Let's wait for native 0.5.x, it's planned for this year and will bring the Threads support.

[sequencer]

Seem scala-native/scala-native#3242 is already landed?
Any plan for os-lib to support os.proc? This might be the last piece to bring scala native become useful.

[lefou]

Haoyi already started something: #257