Unify links and foreign_links

[brson]

It is claimed these two concepts can be unified with specialization. I don't know what that entails.

[andy128k]

Link is specialized by error type and error kind type while foreign_link is specialized by error type only. So there is a temptation to make a foreign link as a special case of a link. I've made an attempt here master...andy128k:feature/unify_links_and_foreign_links

My first impression is that this leads to the invention of extra macros and less readable and less maintainable code. This is caused by limitations of rust's macro tokenizer (and, BTW, this is the same reason why macro impl_error_chain_kind is bloated).

Even if this unification would be successful it will bring a feeling of "magic" and implicitness to library's API.

I would like to convince you, guys, to keep interface (and implementation) as-is. Or even move forward and migrate to proc_macro.