Derive `Copy` on fixed-size types like `Hash`, `Uint256`, `AccountId`

[tomerweller]

What

The code generator derives Copy on C-like #[repr(i32)] enums but never on structs, typedefs, or union enums - even when all their fields are Copy.
This means types like Hash, Uint256, PublicKey, AccountId, and MuxedAccount are Clone-only despite being small fixed-size values whose entire composition chain is [u8; 32] and other primitives.

Why

These types are frequently used in downstream code. Without Copy, every assignment, key construction, map lookup, or pattern-match extraction requires an explicit .clone(). In practice this leads to hundreds of gratuitous clone calls in any non-trivial Stellar Rust project.
Adding Copy is backward-compatible — existing .clone() calls continue to compile.

[leighmcculloch]

Should be pretty easy to add. I would like to merge the new XDR generator before making this change because this will be a change that we'll probably make in the XDR generator.