Support Result<Self, E> returning ink! constructors

[Robbepop]

Currently it is not possible to return anything but Self in ink! constructors. This is an artificial limitation created to make things a bit easier, however, technically as well as from a design perspective nothing speaks against introducing fallible ink! constructors. They will behave in the same way to fallible ink! messages in that they will revert state in case Result::Err is returned.

Mainly ink_lang_ir as well as ink_lang_codegen crates are required to be modified in order to implement this feature.

After this change ink! constructors can either return Self or Result<Self, E> where E is some error type:

use ink_lang as ink;

#[ink::contract]
mod contract {
    #[ink(storage)]
    pub struct Contract {
        value: Balance,
    }

    enum Error {
        TooMuchInitValue,
    }

    impl Contract {
        #[ink(constructor)]
        fn new() -> Self {
            self.value = 0;
        }

        #[ink(constructor)]
        fn with_balance(balance: Balance) -> Result<Self, Error> {
            if balance >= 1000 {
                return Err(Error::TooMuchInitValue)
            }
            Self { value: balance }
        }
    }
}