This is another approach for providing access to stdin/out/err via
libimagrt::runtime::Runtime.
The Runtime object does configure which output gets returned (stdout if
stdout is a tty, else stderr).
With this we can change libimagrt to read/write the store from/to
stdin/stdout without the user noticing that she does not write to stdout
but stderr.
Reading from stdin is not possible then, though.
Matthias Beyer