Improve detection and enforcement of is_avalanching trait

[vmilea]

First of all, thanks for making this awesome library!

Is your feature request related to a problem? Please describe.

The mechanism for tagging high quality hashes could be improved a bit, so it's easier to integrate in existing code bases.

Describe the solution you'd like

1. I prefer to specialize std::hash, but the primary template ankerl::unordered_dense::hash which falls back to std::hash doesn't currently propagate the is_avalanching trait. So it won't be detected unless hashes are moved to ankerl::unordered_dense namespace.
2. In my project, all hashes are supposed to be high quality, but it's easy to forget to add the is_avalanching trait. It would be nice to have a require_avalanching policy on the container that can be asserted at compile time.
3. An additional assume_avalanching policy sounds a little dangerous, but would avoid having to litter code with is_avalanching traits.

Describe alternatives you've considered

My approach for (2) is to wrap the default hash type:

template <typename Hash>
struct require_avalanching : Hash
{
    static_assert(requires { typename Hash::is_avalanching; });
};

// alias with default policy
template <class Key, class Hash = require_avalanching<ankerl::unordered_dense::hash<Key>>, ...>
using hash_set = ankerl::unordered_dense::set<Key, Hash, ...>;

[martinus]

Hi @vmilea, these are all very sensible points. I'll see that I can fix all of them. About (2), I'm thinking whats the best way to implement this is. E.g. I can provide typedefs for the maps in a custom namespace, or globally enable the check with a macro. I then also need a way to opt out, so a way to specify that a hash is not avalanching (maybe with is_not_avalanching or something like that). That also needs to work when the hash can't be modified.

[vmilea]

I think an extra template parameter similar to Bucket would suffice. The user can add typedefs as needed.

A basic enum may work:

enum class hash_policy {
    assume_avalanching,
    require_avalanching,
    detect_avalanching,
};

But a struct HashAdapter would be more powerful. Then the user can create their own adapter, with the escape hatches they desire, even when a hash can't be modified.

[vmilea]

To be clear: the HashAdapter wraps the hash, but is a distinct template parameter on the container. So if the user changes the Hash, the avalanching policy continues to be enforced. Unlike the "alternatives you've considered" code.