reduce arena overrides; tidy up organization of default control macros; eliminate arena casts by directly relying on MD_IMPL_Arena in header

samples/node_errors/node_errors.c

@@ -10,7 +10,7 @@ int main(int argument_count, char **arguments)
     MD_ThreadContext tctx;
     MD_ThreadInit(&tctx);
     
-    arena = MD_ArenaNew(1ull << 40);
+    arena = MD_ArenaAlloc(1ull << 40);
     
     // NOTE(rjf): Parse all the files passed in via command line.
     MD_Node *list = MD_MakeList(arena);