Added some labels to the productions.
Made all ADTs in the AST module public.
Implemented some functions for converting a metaxa description to a
Rascal description. It is unfinished - enough is implemented only to
translate the most basic concepts and structures - and there are known
bugs, as indicated by the `TODO` comments.

The bulk of the work is done through translating an AST representation
of the metaxa description to an AST representation of a syntax
module. Translating this further to a textual Rascal syntax representation is done
through a single function, which is basically just a wrapper around an
imported function.
Changed a label on one of the productions. I was just simply paranoid
about name clashes with the constructors in the syntax module.