Improving Enum Variant definitions

[nicolaassolini-qntm]

Enum variant are declared via dictionary, where key are the variant
fields and values are types;
e.g. variant = {"a": int, ...}
We currently do not support:

• multi assignment: a = b = 1 are not supported
• inline assignment e.g. v1, v2 = {}, {}
• variant=function(...)? [this is more a metaprogramming feature]