Support comparing SDK objects across Env's

[leighmcculloch]

Feature request

For writing differential tests it is important that objects like a Vec, or Address, or any other SDK value, can be compared across Envs. This is because ideally when you are doing differential testing you can start both sets of test cases running in their own Envs so that things like Addresses, IDs, etc are all identical for easy comparison.

I'd like to be able to do this:

#[contract]
struct Token;
#[contractimpl]
impl Token {
    // ...
}

mod token {
    soroban_sdk::contractimport!(file = "token.wasm");
}

#[test]
fn test_differential() {
    let (auths1, events1, ledger1) = {
        let env = Env::default();
        let admin = Address::generate(&env);
        let id = env.register(Token, (&admin,));
        let token = TokenClient::new(&env, &id);

        let a = Address::generate(&env);
        let b = Address::generate(&env);
        token.mock_all_auths().mint(&a, &10);
        assert_eq!(token.balance(&a), 10);
        assert_eq!(token.balance(&b), 0);

        token.mock_all_auths().transfer(&a, &b, &2);
        (env.auths(), env.events().all(), env.to_ledger_snapshot())
    };
    let (auths2, events2, ledger2) = {
        let env = Env::default();
        let admin = Address::generate(&env);
        let id = env.register(token::WASM, (&admin,));
        let token = TokenClient::new(&env, &id);

        let a = Address::generate(&env);
        let b = Address::generate(&env);
        token.mock_all_auths().mint(&a, &10);
        assert_eq!(token.balance(&a), 10);
        assert_eq!(token.balance(&b), 0);

        token.mock_all_auths().transfer(&a, &b, &2);
        (env.auths(), env.events().all(), env.to_ledger_snapshot())
    };
    assert_eq!(auths1, auths2);
    assert_eq!(events1, events2);
    assert_eq!(ledger1, ledger2);
}

Today, if you run the above, it'll result in a panic when comparing the auths1 and auths2, and events1 and events2. The panic occurs because of the same env check here, and in similar places in other types:

rs-soroban-sdk/soroban-sdk/src/address.rs

Lines 81 to 88 in f0e653e

	impl Ord for Address {
	fn cmp(&self, other: &Self) -> Ordering {
	self.env.check_same_env(&other.env).unwrap_infallible();
	let v = self
	.env
	.obj_cmp(self.obj.to_val(), other.obj.to_val())
	.unwrap_infallible();
	v.cmp(&0)

The check is reasonable, because the cmp functions of all the sdk types calls the env and asks the env to do the comparison.

When the cmp functions are called, if currently not building for wasm, the code could check if currently in the same env, and if not, convert the sdk type to it's XDR equivalent, and compare those.

What alternatives are there?

To expect people to do differential testing inside the same env, but that seems unrealistic. It would not be possible to just simply compare two auths or two events objects, becauses the addresses would be different.