refactor: process handling (#8113)

Refactor process handling. This is has two purposes:

* Simplify the implementation and make it easier to follow
* Prepare it for making it friendly for action runners

At a high level, the refactoring does the following:

* Introduce a type for a running process [Process.t]
* Introduce a type for the result of running a process
  [Process.Result.t]
* Introduce a type for handling stdout/stderr without accidentally
  forgetting to clean it up or reading it after they've been deleted.

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>