use package system-argv0 to find IHaskell binary instead of $PATH

This allows you to for example use ./dist/build/IHaskell/IHaskell.
However, things can still be confusing in the sense that your ihaskell
might be using code not from the particular binary you started the
notebook with because there is still only one
.config/ipython/profile_haskell/ipython_config.py