thread context scratch system; converge on transition idiom in sample code

samples/toy_language/toy_language.c

@@ -182,6 +182,9 @@ EvaluateScope(NamespaceNode *ns, MD_Node *code)
 
 int main(int argument_count, char **arguments)
 {
+    MD_ThreadContext tctx;
+    MD_ThreadInit(&tctx);
+    
     arena = MD_ArenaNew(1ull << 40);
     
     //- rjf: parse command line