rustdoc: Add source links for impl

[SimonSapin]

When documenting traits, rustdoc lists known implementations. From there it links to both the trait and the type it’s implemented on, which both (usually) have an [src] link.

However, the source code of an impl is not always next to either of them. Could each impl have an [src] link as well?

For example, it’s hard to go from http://doc.rust-lang.org/std/fmt/trait.Binary.html to https://github.com/rust-lang/rust/blob/master/src/libcore/fmt/num.rs , even grep doesn’t help.

[lifthrasiir]

Duplicate for #15744 I think?

[SimonSapin]

Indeed, sorry for the duplicate.