Add coconut-specific @:forward

[gene-pavlovsky]

I think coconut could benefit from it's own extended version of tink_lang's @:forward

Limitations of using @:forward now:

• Doesn't generate an observable for the property
• If the target field's properties allow write access (e.g. a simple VO with plain vars, marked @:observable), setters will be generated (on the other hand it can be argued that the VOs should be properly defined as immutable)
• Haven't tried this: if properties are forwarded to a nested coconut model, is every kind of property forwarded correctly?

[serjek]

proposition to add a @:proxy meta or something like this to forward observability for given (or all) props for nested models.

current workaround is a bit cumbersome:

@:observable var nested:NestedModel; //can change over time so binding to `model.nested.observables.prop` will not work anymore  once it is changed
@:computed var prop:NestedProp = nested.prop;
...