Add basic tuple and value fetching from queries.

Add safeties for =destroy hooks.

FossilOrigin-Name: 2fae5297a0d0598cc3580777688b4f4307de008d4f379d2fb224c8a74cb9b708

src/kuzu/types.nim

@@ -1,14 +1,16 @@
 # vim: set et sta sw=4 ts=4 :
 
 type
-    KuzuDBObj = object
+    KuzuDatabaseObj = object
         handle*: kuzu_database
         path*: string
         config*: kuzu_system_config
-    KuzuDB* = ref KuzuDBObj
+        valid = false
+    KuzuDatabase* = ref KuzuDatabaseObj
 
     KuzuConnectionObj = object
         handle*: kuzu_connection
+        valid = false
     KuzuConnection* = ref KuzuConnectionObj
 
     KuzuQueryResultObj = object
@@ -18,8 +20,21 @@ type
         num_tuples*:     uint64 = 0
         compile_time*:   cdouble = 0
         execution_time*: cdouble = 0
+        valid = false
     KuzuQueryResult* = ref KuzuQueryResultObj
 
+    KuzuFlatTupleObj = object
+        handle*:     kuzu_flat_tuple
+        num_columns: uint64 = 0
+        valid = false
+    KuzuFlatTuple* = ref KuzuFlatTupleObj
+
+    KuzuValueObj = object
+        handle*: kuzu_value
+        valid = false
+    KuzuValue* = ref KuzuValueObj
+
     KuzuException* = object of CatchableError
     KuzuQueryException* = object of KuzuException
+    KuzuIndexException* = object of KuzuException