Implementing new exchanges as independent crates

[HereticSK]

Under the current structure, implementing a new exchange requires merging new codes into the codebase, such as creating a new crate in the exchange, adding a new variant to the ExchangeId enum.

But there are cases where users may want to implement some exotic exchanges that are not interested to other users. I hope we can have a better way to factor out exchange specific codes, so users can implement whatever exotic exchanges as they like, without affecting the whole project.