Deferred is useful for "at-most-once" computation of expensive values.

Currently, it wraps an std::function. Previously, the overhead of std::function in hot paths has been found to be significant.

Deferred does not always require the type erasure of std::function.

Therefore, refactor Deferred to allow storing arbitrary callables. This makes Deferred more flexible, while existing uses which do require type erasure can explicitly use Deferred<std::function<...>>.

With this change, the following does not require an std::function

auto deferredValue = Deferred([&](){
    // Do something expensive
    return thing;
});
...

void maybeUseExpensiveValue(auto func) {...}

maybeUseExpensiveValue(deferredValue);
maybeUseExpensiveValueSomewhereElse(deferredValue);