Inconsistent {:newType} and {:newType true}

[MikaelMayer]

To be consistent with other attributes, {:nativeType} and {:nativeType true} should do the same thing.
Currently, the documentation says:

• {:nativeType} - With no parameters it has no effect and the NewtypeDecl
  will have its default behavior which is to choose a native type that can hold any
  value satisfying the constraints, if possible, otherwise BigInteger is used.
• {:nativeType true} - Also gives default NewtypeDecl behavior,
  but gives an error if base type is not integral.