Add `encase` trait implementations

[teoxoy]

I would like to remove trait implementations on foreign types from encase for the reasons listed in teoxoy/encase#94.

They are currently hosted here: https://github.com/teoxoy/encase/blob/fcc83d706f4a51bfbe22c9901973f1c2fa806526/src/impls/smallvec.rs

Given that I could only find one project (and one if its hard? forks) using the smallvec feature of encase, would the smallvec developers be willing to host the trait implementations?

[mbrubeck]

Yes, we could add the impls to this crate.