Add MentalExistent hierarchy of classes.

These differ from the MentalEntity class hierarchy in that
MentalExistents are a narrower subset of MentalEntities. MentalEntities
refer to all mental content that needs to be persistently stored to
represent cognitive and personna state.

MentalExistents are content of the mind that specifically represents
existents. I.e: perceptual data, whether structural or implicative.

smocore/include/mentalEntity.h

@@ -3,10 +3,24 @@
 
 namespace smo {
 
+/*
+ * MentalEntities refer to all mental content that needs to be persistently
+ * stored to represent cognitive and personna state. This includes such things
+ * as value judgements, comparators, logical ops, DB links, etc.
+ *
+ * Mental Existents are a narrower subset of MentalEntities that represent
+ * implicit existents and perceptual data, both implexed and artificed.
+ */
 class MentalEntity
 {
 public:
     using Id = uint32_t;
+
+public:
+    MentalEntity(const Id id) : id(id) {}
+
+public:
+    const Id id;
 };
 
 } // namespace smo