Short arrays generation in var_export function

[jorgsowa]

PoC for short ararys generation in var_export function.

I want to check whether this idea makes sense. If so then I can polish the PR and finish tests.

[bukka]

This has been discussed before: https://externals.io/message/109415 . But looks like the RFC never got to a vote at least it looks like it from its status. I would personally prefer version with multiple flags as proposed by Sara.

[jorgsowa]

@bukka thank you for this link. I wasn't aware of the existence of topic.

[Ayesh]

I'm afraid this might have a sour taste because the array() syntax is still valid, and as far as I see, we have not mentioned anything to encourage [] syntax over array().

I like that this PR does not change to [] syntax by default, and as Jakub mentioned, I'd love to have multiple flags as well. We are yet to recover from the setcookie parameter hell, so having multiple flag support from the beginning makes more sense.

As for BC, I wouldn't bother too much because PHP 5.4 is wayyyy too old for us to care about.