NSInternalInconsistencyException occurs when shellOut is run too many times #79

I've already opened a PR for this issue, but wanted to open an issue for documenting purposes.

Copy-pasta from the PR description:

I have a Swift CLI that utilizes ShellOut to interact with binary executables. When I use shellOut too many times, I sometimes hit an exception that looks something like:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Failed to set posix_spawn_file_actions for fd -1 at index 0 with errno 9'
*** First throw call stack:
(
	0   CoreFoundation                      0x00000001859672ec __exceptionPreprocess + 176
	1   libobjc.A.dylib                     0x000000018544e788 objc_exception_throw + 60
	2   Foundation                          0x0000000186a92eb4 -[NSConcreteTask launchWithDictionary:error:] + 4240

I've been able to determine this is due to the Pipe objects from ShellOut not being closed properly.

I've included an example project that demonstrates the issue:
NSExceptionExample.zip

Steps:

1. Unzip the example library and cd NSExceptionExample
2. Run swift run NSExceptionExample
3. While that command is still running, in a separate terminal window, run ps aux | grep NSExceptionExample and take note of the PID.
4. Run lsof -p <pid> | wc -l
5. Notice how this number is quite large. Also notice that running again shows the number increasing.
6. If the swift command runs long enough, you'll hit an exception like the above.