Separate "setup" and "work" code in runtime

[andreypopp]

Some functions (like f/) do some "setup" (alloc the container for the result) and then do some "work" (fill the container). As per my understanding we should split those two phases into separate methods so we'd allow Julia to compile specialised methods for "work" code.

https://docs.julialang.org/en/v1/manual/performance-tips/#kernel-functions