Use anonymous records for AST nodes

[jubnzv]

Anonymous records in constructors would help remembering the meanings of AST node components and also with standardizing the naming scheme of AST node components.

Here is an example: https://github.com/mtt-lang/mtt-lang/blob/master/core/Ast.ml#L37

Probably, we could automatically rename ADT uses in pattern matching with comby.