Rename iter\fn to iter\func

[nikic]

It's likely that fn will become a reserved keyword due to https://wiki.php.net/rfc/arrow_functions_v2, so iter\fn should be renamed to iter\func for version 2.0.

[nikic]

Voting is underway and the RFC is very likely to be accepted, so should move forward here.

[nikic]

Done in 2d0b618. I think in PHP 7.4 you would just use an arrow function for most of what iter\func provides though :)