Make array to string conversion a recoverable error

[hikari-no-yume]

Implements this RFC: https://wiki.php.net/rfc/array-to-string

Does not update any of the numerous tests it breaks. Also, given that engine exceptions passed, it might be better to make this throw an exception instead.

But by making this pull request, I might give the fact this RFC is unimplemented more attention.

[cmb69]

Good catch, Andrea. Note that there is also PR #991 which is basically identical.

BTW: I'm pretty much surprised to see that there are several PRs targeting PHP 7 with pending implementation. If I'm not mistaken 7.0.0beta1 will be tagged tomorrow.

[hikari-no-yume]

So @nikic informed me that the reason this hasn't been implemented yet is that the Exceptions in the Engine RFC passed, so we'd have to make it throw an exception, yet unfortunately we can't allow anything used in __toString to throw exceptions.