Posix.runProcess

I think Posix.runProcess should give access to the eventual exit state of the process, if any. Two possible ways: (1) Change its result to IO (IO ExitCode), the inner action being something that waits for the process to complete (without blocking all other threads!) and returns the ExitCode. (Even better would be to make the result just IO ExitCode but this might be too incompatible for people who rely on runProcess returning at once.) (2) If (1) can't be done, change its type to return something similar to Posix.getProcessStatus.