refactor(userspace/engine): refactor engine interface and internals

This updates the engine to comply and work properly with the newly-introduced
interface design.

Signed-off-by: Jason Dellaluce <jasondellaluce@gmail.com>

userspace/engine/falco_rule.h

@@ -20,9 +20,13 @@ limitations under the License.
 #include <string>
 #include "falco_common.h"
 
+/*!
+	\brief Represents a rule in the Falco Engine.
+	The rule ID must be unique across all the rules loaded in the engine.
+*/
 struct falco_rule
 {
-	size_t id;
+	std::size_t id;
 	std::string source;
 	std::string name;
 	std::string description;