Make #[transaction] to work on functions not returning Result<T, E>

[byronwasti]

Currently, #[transaction] can only be used on async functions which return a Result<T, E>. This let's Balter keep track of the error-rate.

#[transaction]
async fn foo() -> Result<(), Error> {
}

/// Error
#[transaction]
async fn bar() -> MyType {
}

However, certain transactions it might not make any sense to return a Result, and this becomes an unnecessary type restriction. Unfortunately I don't think this is trivial to support. If we have a #[transaction] be anything which returns impl TransactionOutput, then I think the options are limited:

1. Users need to implement this on their types, which is annoying
2. Opt-in to the specialization unstable feature, which I'd like to avoid (in order to special-case just Result)
3. Expand the #[transaction] macro to do more code-gen to make this viable

From what I understand of the issue, (3) is the most viable.