Parse the return type of the function that will be attributed with the `freezable` macro

[ozgunozerk]

Recap: freezable macro would generate a trait, in which, we have the Output type (https://github.com/ozgunozerk/asynction/blob/main/freezable/src/lib.rs#L29).

From the signature of the function we are applying #[freezable] on, we should be able to get the return type, so that we can assign it to the type Output.

For example, from:

fn freezable_complex(begin: u8) -> String {

parse -> String