Possibility for call-by reference

[sebffischer]

I am not sure whether this is desirable but I think it might be worth to consider the possibility for allowing call-by-reference semantics. In C++, e.g., the & in the function signature void mutate(double& x); causes the x to be passed by-reference and is therefore mutable from within the function.

I think there are some arguments in favor of allowing for a standardized interface for reference semantics:

1. In R, data.table is a very widely used library and uses references semantics for performance. So there is demand for such a mechanism. Another prominent example is R6, which definitely also has it's use-cases, e.g. for logging via the lgr package.
2. If one does not provide a proper mechanism for reference semantics, people will still be able to achieve this in a probably more hacky way.
3. If one provides an Environment structure, the language already has references semantics so extending this to other data types does not seem to big of a stretch.

The argument against references semantics I think is that it introduces a lot of complexity, consider e.g. the whole shared vs exclusive reference mechanism in rust.