Added some simple language definitions defined in both rascal and
metaxa. I've yet to define the exp2 language for metaxa (only for
rascal), since I don't yet know how I should define precedence of
operators among other things.
Changed nesting of `sort Program`; made it the outermost sort. This
seemed more consistent with the grammar in the magnolia-eclipse, as
"Program" is the start production of that grammar.
Removed some `;` that seemed superfluous (it still parses). The
inclusion of those `;` in the grammar seem unnecessary in themselves,
for that matter.