Allow for implicit constraints in `DataClass` function

[RobertDober]

Now

    DataClass(:a, b: nil) # A default

Then

All cases not matching the patterns described below remain default declarations

   DataClass(:a, b: C(String))  # C is Kernel::C -> DataClass::Constraint.new, should C(String) imply a default too?
   DataClass(:a, b: C(String, "")) # Second parameter of C is default
   DataClass(:a, b: -> { _1 > 0 }) # Proc is **always** a constraint 

Maybe better

   DataClass.constrained(:a, b: String)
   DataClass.