we should not be incestuous

[Matthew Fluet]

> The  MLton.random  thing still confuses me: why should this be part of MLton.
> You can just write it using the basis library.

Currently MLton.random does a ffi call to a C function which reads
/dev/random.  I guess I agree with Henry that there isn't any reason that
can't be done with the BinIO, but does raise the related question of where
to put Linux (or more generally, OS) specific functionality.