Enable cloning EntityHashMap and PreHashMap

[cBournhonesque]

Objective

• EntityHashMap, EntityHashSet and PreHashMap are currently not Cloneable because of a missing trivial Clone bound for EntityHash and PreHash. This PR makes them Cloneable.

(the parent struct hashbrown::HashMap requires the HashBuilder to be Clone for the HashMap to be Clone, see: https://github.com/rust-lang/hashbrown/blob/master/src/map.rs#L195)

Solution

• Add a Clone bound to PreHash and EntityHash

[alice-i-cecile]

It would probably be better to also swap the Clone impls on the generic parent structs to a manual clone impl: the lack of "perfect derives" means that it incorrectly requires a Clone bound.

[cBournhonesque]

It would probably be better to also swap the Clone impls on the generic parent structs to a manual clone impl: the lack of "perfect derives" means that it incorrectly requires a Clone bound.

The parent struct (hashbrown::HashMap) actually already as a manual clone impl, but it requires the HashBuilder to derive Clone. See: https://github.com/rust-lang/hashbrown/blob/master/src/map.rs#L195
I will update the PR description to make this clear

[james7132]

LGTM, though one open note about our use of compile tests.

[james7132]

I'm pretty partial to having something similar to static_assertions (it's already in our dependency tree), which includes a assertion that will fail compilation if a type does not implement a trait. I think it should be fine to use it as a dev dependency over these tests which actually need to run when running our test suites.

This is a strict non-blocker and we can tackle this in a separate PR.

[cBournhonesque]

Ah i was thinking of re-using trybuild (https://docs.rs/trybuild/latest/trybuild/) since I see that it's already used in bevy somewhere else, but static_assertions seems to have more powerful checks