Make environment arguments reference-only

[Feliix42]

Currently, environment values are captured as owned references and cloned, if necessary. This raises a number of issues however, ranging from required workarounds (see #29) to bugs due to the fact that ohuac does not know about these changes (see smap::smap_with_envarc_in_loop test case).

When discussing this with Justus, we agreed that enforcing the use of borrowed environment arguments would be a fine thing to do as it would make the constraints to environment arcs transparent.

Enforcing this could become quite hard however, due to the fact that we cannot typecheck a variable name handed to the macro.

It might suffices to just remove any existing workarounds and add a note to the documentation, saying that any environment values have to be borrowed?

[Feliix42]

This also raises the question, whether this should be valid:

fn main(ctrl: &bool, inp: &String) -> String {
    if (ctrl) {
        modify_string_positive(inp)
    } else {
        modify_string_negative(inp)
    }
}

Technically, ctrl is supplied as input to an operator (namely if), which should not happen according to the current implementation. If this should indeed be invalid, I would prefer if ohuac would check invariants like these.