[Feature request] generating BNF from ADTs

[ChenZhao44]

It will be very useful if we can generate BNF from user-defined ADTs automatically.

[Roger-luo]

this should be possible by just walking through the definitions via reflections (the variant_xxx functions), the question is it does not fully compatible with BNF - ADTs do not have syntax information, generating a specific BNF would be possible.