add hss.c module

[gbanegas]

The code in HSS uses the hash-sig package from https://github.com/future-proof-iot/hash-sigs/
The main idea in the hss.c module is that it calls the "api.h" in the package and return the desired functions: Key generation, sign, and verification.

[kaspar030]

The main idea in the hss.c module is that it calls the "api.h" in the package and return the desired functions: Key generation, sign, and verification.

I don't understand the point of these wrappers. hash-sig already exports "sign()" "veriy()" and "keygen()" as global symbols, can't we just use those?
It get's more interesting if libcose were to support multiple backends, with probably duplicate "sign()" etc. in that case, we'd either rename upstream, use gcc's replace functions, or something else.

[bergzand]

It would be good to have some way to wrap these functions to a generic cose wrappers in case we get another library supporting hss-lms. I don't mind these wrappers for now

[kaspar030]

I've used this hss.c as basis for hash-sigs.c of #119.
Thanks @gbanegas!