Key and Value Iterators

[larkwiot]

Hello,

I love these map libraries and am using them extensively, thank you so much for sharing your work!

I am frequently running into the need to iterate over the keys or values separate of the other. Is there any way this could be implemented for this map?

Currently I have implemented utility functions to gather the keys or values into a vector and then use that, but I'd like to do this iteration in a functional programming kind of way (so without a for loop and a map() or transform() instead) and also without taking up the memory to copy all the keys or values into a separate vector.

Let me know your thoughts on this, and thanks again for your hard work!