File IO Builtins

[eignnx]

• [ ] Add builtin [fname][open_fd][mode] which opens the file with mode Mode and unifies OpenFd with the file descriptor.
• [ ] Allow [io_write][stream] to write to integer file descriptors bound to Stream.

[eignnx]

I actually don't think passing around file handles or file descriptors is the best idea. Rust really doesn't like working with raw file descriptors or converting to them from ints.

I think a simpler design for now is to allow output redirection via a built-in like this:

• [ ] [goal][stream][redirection_path] would run Goal but first redirect Stream (either stdout or sterr) to a new file located atRedirectionPath.