`weak_table` optional dependency with adequate `WeakElement` implementation

[AdrienChampion]

We prefer not adding dependencies if we can avoid it, but at least two users expressed interest in integrating with weak_table (#15). While there is a reasonable workaround, actual integration would be much nicer.

This issue is to assess how big the interest for integration is, please let us know here.

[asvarga]

My use case for this was for memoizing a function that takes an HConsed<T> with a WeakKeyHashMap so the table itself wouldn't contribute to the HConsed<T>::elm's reference count. I'm doing something like:

use once_cell::sync::Lazy;
use std::sync::{RwLock, Weak};
use weak_table::WeakKeyHashMap;

pub fn term_to_output(term: &Term) -> Output {
    static MEMO: Lazy<RwLock<WeakKeyHashMap<Weak<ActualTerm>, Output>>> =
        Lazy::new(|| RwLock::new(WeakKeyHashMap::default()));

    if let Some(ret) = MEMO.read().unwrap().get(&term.0) {
        return *ret;
    }

    let ret: Output = todo!();

    MEMO.write().unwrap().insert(term.0.to_weak_ref().upgrade().unwrap(), ret); // yuck
    ret
}

Questions:

• Is there a better way to accomplish this with what hashconsing already provides?
• Can this optional dependency also provide integration with https://crates.io/crates/memoize via weak versions of the structures in hashconsing::hash_coll?

[AdrienChampion]

Regarding caching using weak references, I actually wrote an example in a branch back when I misunderstood your question in #15. It only uses std.