* Move `Process_Listener` to a dedicated package.
* Make `Spawn.Processes.Process` as a wrapper for
`Spawn.Internal.Process` to hide internal details from a user.
* Make `Spawn.Internal.Process` API compatible/equal to
`Spawn.Processes.Process`. Inherit all implementation from
`Spawn.Common.Process` and move common trivial functionality
there.
* Move other logic from Process.Platform packages to corresponding
`Spawn.Internal` package.
* Move `Spawn.Process.Monitor` to `Spawn.Internal.Monitor`.
