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

tests/cpp_build_test.cpp

@@ -8,7 +8,7 @@ int main(void)
     MD_ThreadContext tctx;
     MD_ThreadInit(&tctx);
     
-    arena = MD_ArenaNew(1ull << 40);
+    arena = MD_ArenaAlloc(1ull << 40);
     
     printf("%d\n", MD_CPP_VERSION);