Current state of list comprehison in Nim

[Hlaaftana]

Note that while 
[sugar.collect](https://nim-lang.github.io/Nim/sugar.html#collect.m%2Cuntyped%2Cuntyped)
 is the recommended and builtin way of list comprehension, it is just one macro 
and there are many ways to implement list comprehension. Even though you might 
not be able to implement the syntax that you're used to (like `[(a, b) for a in 
1..5 for b in 1..5]`), I believe you should still be able to get syntax that is 
intuitive enough.