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

samples/static_site_generator/static_site_generator.c

@@ -37,7 +37,7 @@ 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 command line arguments.
     MD_String8List arg_list = MD_StringListFromArgCV(arena, argument_count, arguments);