Distinguished let-syntax for sets and relations. Now write
"let rln x = ..." to define a relation, and write
"let set x = ..." to define a set. The "rln" keyword can be omitted.

Added empty set. Use "let rln x = 0" for the empty relation, and use
"let set x = 0" for the empty set.

Simplified AST to remove unnecessary Konst constructor.