BorshDeserialize shouldn't require `Copy` about borsh-rs

[frol]

I don't think there was a particular reason. It might have been some Rust limitation defining boundaries, but other than that, I don't know what holds us from fixing it.

[lovrozitnik]

Please fix this!

[austinabell]

Looked into this a bit. Very hard to make a simple fix now that there are const generics for the array impls and they don't play nice with MaybeUninit arrays (trying to remove the clone bound altogether). Also noticing that there is a very unnecessary Default bound on the array to deserialize. This is only used to write default values to the array before writing over it.

Also not straightforward to initialize default array with the generic type. Seems like the only way to solve this is with some possibly janky unsafe code.

Doesn't really seem worth it to do a hacky solution to just switch Copy to Clone since it comes with some other unideal side effects. Opening PR soon