This is faster and we don't need to use global Io extra time.

This is just managed version of array_hashmap.String.
Making it unmanaged tells uset in api what passing it as parameted doesn't modify it. 

I think we can mitigate a bit more code by adding
initElement(arena, &.{.{key1, value1, .{key2, value2}}}); init method to that datastructure.
It already exists on StaticStringMap so i don't think anyone would be agains it.

This gives users more flexability on where to actually put buffer and how large to actually make buffer.
And also makes Message non intrusive data structure which allows nicer initialization.
This also allows to remove default field values which removes footgun of incorrect initializations.

They actually just don't test anything.