Promote the warning of array_key_exists() to exception

[kocsismate]

I am not exactly sure if these promotions fall into the scope of the Engine Warnings RFC (are these errors suppressed commonly?), so feel free to close the PR in this case.

Also, I made two more promotions in the standard/array.c file, but these doesn't seem likely to happen (one of them is in an apparently not used macro and the other should never happen according to its comment).

[nikic]

See #4572.

[kocsismate]

Yes, I should have remembered about this PR (or at least checked for it). :D OK, not doing warning promotions certainly makes sense for count(), but what about array_key_exists()? It seems to be a less severe BC break. I can understand if it is also a no-go.

[nikic]

@kocsismate Chaning array_key_exists is generally fine, but please make sure it has consistent type behavior with isset($array[$key]) first. I remember there were some discrepancies (maybe with handling of float keys?)

[nikic]

That is, the argument should not be "string or integer", it should work based on the normal array key coercions that are used everywhere else where array accesses are performed. Only arrays and objects should result in a TypeError.

[kocsismate]

@nikic I tried my best to do what you suggested.