Put the literals into a separate namespace; resolves #31 (#32)

tests/sqlite/test_operations.cpp

@@ -27,6 +27,7 @@ TEST(sqlite, test_operations) {
            .id = 3, .first_name = "Maggie", .last_name = "Simpson", .age = 0}});
 
   using namespace sqlgen;
+  using namespace sqlgen::literals;
 
   struct Children {
     int id_plus_age;