Feat: add return type assertion

[indirection42]

Design

• Generate multiple instances with specific types for a single "polymorphic" XCQ program.

For example:

#[xcq_api::program]
#[xcq_api::extern_type(AssetId,AccountId,Balance)]
mod sum_balance {
    #[xcq::call_def(extension_id = 10588899351449456541u64, call_index = 1, )]
    fn balance(asset: AssetId, who: AccountId) -> Balance {}

    #[xcq::entrypoint]
    fn sum_balance(calls: Vec<BalanceCall>) -> Balance {
        let mut sum = 0;
        for call in calls {
            sum += call.call();
        }
        sum
    }
}

will generate multiple XCQ program instances including the following instance:

#[xcq_api::program]
mod sum_balance {
    #[xcq::call_def(extension_id = 10588899351449456541u64, call_index = 1)]
    fn balance(asset: u32, who: [u8; 32]) -> u64 {}
    #[xcq::entrypoint]
    fn sum_balance(calls: Vec<BalanceCall>) -> u64 {
        let mut sum = 0;
        for call in calls {
            sum += call.call();
        }
        sum
    }
}

• Front-end codes will choose the correct prebuilt XCQ program according to the host chain's type, and construct corresponding data.

Future work:

• A script to generate multiple instances of a single XCQ program
• Frontend codes to detect the host chain and choose the correct XCQ program instance.

[xlc]

maybe is better to specify extension_id by hex? e.g. 0x12345u64 so that it is shorter

[xlc]

use match instead of multiple if else if