Report error when declaring token as NTA

[Jesper Öqvist]

JastAdd currently allows declaring tokens as NTAs. This is an unintended feature which should not be used for the following reasons:

1. Tokens can have non-AST types and non-AST types can't be NTAs.
2. In principle there is no difference between a computed token and an ordinary attribute. It is better to just use an attribute instead.

NTA tokens happen to work in some cases just because the code generation accidentally doesn't produce broken code.