Suggestion: Make fallback_allocator::stateful_allocatorator_available() public

[bigerl]

https://github.com/LLNL/metall/blob/1f6c027680ea89bf92bd9a5255e006c044aa078f/include/metall/container/fallback_allocator.hpp#L217-L219

We use the fallback_allocator in an algorithm that behaves differently based on weather the stateful allocator is used or not. ATM, we just re-implement the private_stateful_allocatorator_available() in our code.

I would suggest to make a public stateful_allocatorator_available(). Shouldn't hurt because the interfaces to calculate the result are public anyway and might come in handy for others.

PS: Is allocatorator a pun or a typo?