Correct handling string return values

[mohammadfawaz]

Two things:

• Fixing the size of a String.
• Treating Strings of size 1 word or less as copy types. We might have to do the same for structs and other complex types when they're small enough to fit in registers.

The following contract, for example, now returns the correct receipts and can be tested from the SDK.

contract;

abi MyContract {
    fn get_small_string() -> str[8];
    fn get_large_string() -> str[9];
}

impl MyContract for Contract {
    fn get_small_string() -> str[8] {
        let my_string: str[8] = "gggggggg";
        my_string
    }
    fn get_large_string() -> str[9] {
        let my_string: str[9] = "ggggggggg";
        my_string
    }
}

The SDK has a test similar to the above that should be re-enabled (cc @digorithm). For now, I can't really check this from a script because I can't check what was returned from the contract call against a literal String using an assert. Relevant: #1291

[mohammadfawaz]

If we go with this, then we should probably make it clear, in the language, that types that "fit in a word" are always copy types. That includes things like str[1], struct { u64 }, struct { struct { u64 } }, array[1], and so on. It is a bit weird, but I think this would make the specs a bit simpler where the optimization that we've been talking about is actually baked in the language and has to happen.

[otrho]

I think you're right. We'd absolutely need regression tests to enforce it for all our types too.

[otrho]

I think you're right. We'd absolutely need regression tests to enforce it for all our types too.