Use new internal functions for pinning

[carstenbauer]

Currently, we pin threads by using @tspawnat or @threads :static to run libuv functions on specific threads. In Julia 1.11, there will be two new functions, jl_getaffinity and jl_setaffinity, which we probably should use instead (because it's cleaner).

Also, this will allow us to pin GC threads easily (#73).