support stable version of Rust

[sehz]

We rely on on Rust's #![feature(specialization)] which works only on nightly. This is used by Encoder and Decoder traits to implement default parser for Vector.

Revisit basic Encoder/Decoder implementation in the kf-protocol-core to see if we can get rid of need for ```default`` implementation logic. Specifically

• Option
• ....

Rust issue

[sehz]

Possible workaround: http://lukaskalbertodt.github.io/2019/12/05/generalized-autoref-based-specialization.html