reduce usage of IntoIterator<Item = Expr> in logical plan builder window fn

[jimexist]

I am not sure we should use IntoIterator<Item = Expr> for every field with 6 fields. This will produce a version of the compiled function for every combination, which IMO adds an unnecessary compile time and binary size.

IntoIterator is more relevant when we want to avoid an extra allocation; these are very small vectors.

Originally posted by @jorgecarleitao in #334 (comment)