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

tests/unicode_test.c

@@ -16,6 +16,9 @@ void run_test_on_string(MD_String8 string)
 
 int main(void)
 {
+    MD_ThreadContext tctx;
+    MD_ThreadInit(&tctx);
+    
     arena = MD_ArenaNew(1ull << 40);
     
     // TODO(allen): throw more at this.