feat: add an MashMap::entry_(mut_)iter api

[llogiq]

To avoid borrowing errors when returning as an impl Iterator, I needed a method that would consume the key. So I added it, and a mutable version to complete the API.

[imartayan]

Thanks for the PR!
I agree that this method is useful, ideally I'd like to have a method similar to HashMap's entry, returning an iterator of Entry instead of Bucket.
Would this approach suit your needs?
Also, could you give an example where you need to consume the key?

[llogiq]

Sure. Giving a key is useful to build a function that returns an impl Iterator<Item = Foo> + '_ without needing to borrow into both the key and the MashMap. E.g.

fn iter(&self, key: u32) -> impl Iterator<Item = &str> + '_ {
    self.map.entry_iter(key).map(|s| &s)
}

With the current get_iter method, this would run into a "returning a value borrowing a temporary" error.

Also in async code, borrows can be problematic because the compiler has a harder time figuring out the lifetimes.