Add conversions from File and Child* handles to Stdio

Add conversions from File and Child* handles to Stdio

`Stdio` now implements `From<ChildStdin>`, `From<ChildStdout>`,
`From<ChildStderr>`, and `From<File>`.

The `Command::stdin`/`stdout`/`stderr` methods now take any type that
implements `Into<Stdio>`.

This makes it much easier to write shell-like command chains, piping to
one another and redirecting to and from files.  Otherwise one would need
to use the unsafe and OS-specific `from_raw_fd` or `from_raw_handle`.