freely mix comprehensions, spread arguments, and listed arguments

[gavinking]

Currently, for no particularly good reason, a spread argument or comprehension must be the very last argument in a list of arguments. For #807 we should remove this limitation, allowing all three kinds of arguments to be freely mixed.

[lucaswerkmeister]

I noticed that you can’t spread arguments in a comprehension:

{Result+}|NoResult ret = getResults();
{Result*} resultsOrNothing = { if (is {Result+} ret) *ret };

isn’t allowed, you have to emulate it with an extra for comprehension clause.
Would that be part of this issue?

[gavinking]

@lucaswerkmeister that is #860.