Replace ZeroInit with bytemuck::Zeroable

[yvt]

r3_core::utils::ZeroInit should be replaced by bytemuck::Zeroable. The latter is derivable (which helps us remove some unsafe impls) and has a better Rust ecosystem compatibility.

Zeroable is missing the following implementations:

• core::atomic::Atomic{Bool,{U,I}{8,16,32,64,size},Ptr<impl Sized>} (Zeroable should be implemented for Atomic* Lokathor/bytemuck#74)
• Option<fn(Args...) -> R>
• Option<&[mut ]_>
• UnsafeCell<impl Zeroable> (Implement Zeroable on UnsafeCell<impl Zeroable> Lokathor/bytemuck#148)
• [impl Zeroable + Sized] Zeroable requires Sized

[yvt]

Merged in b6de1c0