Kelvin was working on pure-Swift versions. I don't know what he plans to do with it (if anything); maybe he'd welcome you expanding on that code.
As it says in the thread, the core team would prefer to keep the standard library minimal, and for such functions to evolve in 3rd-party open-source libraries. Once the community settles on a defacto "standard" library for something like trig, we can talk about absorbing that functionality and design in to the language-wide standard library.