Multi-threaded operators evaluation

[lostella]

It would be nice to add multi-threading to some of the operators e.g. using @threads, see https://docs.julialang.org/en/v1/base/multi-threading/index.html#Base.Threads.@threads

Moreover, Julia 1.3 will come with composable multi-threading which should allow for more flexibility using @spawn, see https://julialang.org/blog/2019/07/multithreading