Add datalog infrastructure for min aggregation function

This patch adds an instruction to the datalog interpreter and
constructs a new AST node for min aggregation functions.

The compiler is currently still work in progress and depends on
changes made to the handling of simple joins and the preprocessor.

Signed-off-by: Alex Horn <t-alexh@microsoft.com>

src/muz/base/dl_rule_set.h

@@ -179,6 +179,7 @@ namespace datalog {
         void compute_deps();
         void compute_tc_deps();
         bool stratified_negation();
+
     public:
         rule_set(context & ctx);
         rule_set(const rule_set & rs);