Support KeyValueIterator properly

[back2dos]

Currently for (key => val in target) is treated in a way that doesn't leverage KeyValueIterator, which sometimes is slower and sometimes doesn't even compile.

[markknol]

I do <for ${idx => item in chordsInScale}> but I get this error:

It looked like it is supported, but there's something wrong with it. Then I found this issue.

[serjek]

older versions of coconut that use tink_lang by default do not allow for (k=>v in map) syntax in models because of that.