Add try_insert_with_key that accepts a fallible closure

[caelunshun]

Adds a new try_insert_with_key method to SlotMap, DenseSlotMap, and HopSlotMap. This method accepts a closure returning a Result. If the closure returns an error, try_insert_with_key leaves the slotmap untouched and returns the error.

This is useful when value construction is fallible.

[orlp]

If you address my comments in all three variants this seems like a nice addition.

[orlp]

We might get better code generation if we do

#[derive(Debug)]
enum Never { }
self.try_insert_with_key::<_, Never>(move |k| Ok(f(k))).unwrap()

[caelunshun]

Sounds good. I've addressed your comments and used the Never trick.