File descriptors as arguments

[andrewchambers]

Because janet shell is typed, we can do something like:

cat (something returning core/file) -> cat /dev/fd/63

This can work in tandem with coprocesses.

questions, does this work on all platforms? if not, can we make it work using temporary mkfifo, this probably would involve a list of paths to unlink on job cleanup.

Should we close the descriptor after forking in the parent?