Use `core` and `alloc` over `std`

[Jondolf]

As bevyengine/bevy#15281 is getting merged, we should also explore making Avian less dependent on std. This could make it work on less common platforms such as the Nintendo Switch and likely many other consoles, as well as retro devices.

We should try to rely only on core and alloc where possible. To enforce this, we could add the following lints, like in bevyengine/bevy#15281:

• std_instead_of_core
• std_instead_of_alloc
• alloc_instead_of_core

This would be an important step towards no_std support.

I have not yet investigated how challenging this would be to implement or what blockers we may have, but I am opening this to track progress.