Haxe 4 / dev branch support

[Gama11]

Because of HaxeFoundation/haxe#6380, json2object needs an update to support Haxe 4. Ideally, the haxe_ver defines is checked (example) to support both Haxe 4+ and 3.4.

[ibilon]

Tried in 07e52e0, it fixes the issue but I'm seeing some inconsistencies in the tests on different targets.
Some say null should 0 or null should be [] but not all.

Also is the haxe_ver check necessary? There's either the null abstract or null typedef, so there shouldn't be any conflict in checking both.

[Gama11]

Hm, right, might not be necessary.

[ibilon]

Done in 6d0c25e

[Gama11]

Hm, something seems to be broken.. will look into it later. https://travis-ci.org/vshaxe/vshaxe#L677

[Gama11]

Nvm, that was actually an issue with the macros in haxe-formatter, which also needed an update.