Warning for using Unduckable classes as return types

[eyhlin]

Classes without a 0 argument constructor or a default constructor are unduckable.
Public procedures that return such classes will be generated as blocked procedures, but the user may be unaware of it.
An improvement will be to let the compiler to provide warning messages when such public procedure are declared.

Example code:

package test;
public class BooleanC {
    private boolean v;
    public BooleanC(boolean v) { this.v = v; }
    public boolean value() { return v; }
}

package test;
@Capsule 
public class Test{
    public Boolean proc(){ return new BooleanC(true);}
}

The public procedure proc() in Test will be a blocking procedure.