Expand `impl Encode for Vec<String>` to `impl Encode for Vec<E: Encode>`

[Eugeny]

Would be useful for Vec<&str> and key exchange Vec<Algorithm> lists for example.

[tarcieri]

@Eugeny can you open a PR?

[Eugeny]

I would but my PRs from November are still hanging open :(

[tarcieri]

I will get to them soon, I am just going through a very, very large backlog