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/rel/dl_relation_manager.h

@@ -251,6 +251,9 @@ namespace datalog {
             return mk_join_fn(t1, t2, cols1.size(), cols1.c_ptr(), cols2.c_ptr(), allow_product_relation);
         }
 
+        table_min_fn * mk_min_fn(const table_base & t,
+            unsigned_vector & group_by_cols, const unsigned col);
+
         /**
             \brief Return functor that transforms a table into one that lacks columns listed in
             \c removed_cols array.